I REALLY love this mutation and use it as my go-to for whenever I play any custom campaigns on singleplayer because it flips the normal gameplay meta for l4d2 on its head. Usually, the game teaches you (through use of the director and random hordes particularly) to run ahead and get through the level as fast as possible before you run out of health or ammo, but this mutation encourages checking every nook and cranny for equipment that can be your crutch through sticky situations like a surprise map-side horde trigger, or a tank, or a gauntlet run. As a result, I move much slower through the maps and thus am more absorbed in the environment around me.

My only complaint is that I wish it still included special infected because of their ability to suddenly make the survivor(s) switch up their strategy on the fly, and it helps to break up the tedium of vanilla common slaying which this mutation dials up to 11, thus making it feel tedious much faster as a result. Obviously, though, the specials would need to be heavily modified to make them fair in a solo or duo co-op scenario.
Here's my recommendations to rebalance them in ways I think would make adding them to Overrun an even better experience without sacrificing the principle of the mutation, which is of course to patiently scavenge, hoard equipment, and take your time.

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1) spawn VERY infrequently, anywhere between once every 6-11 minutes.
2) have their scratch damage do twice as much versus a common infected on any given difficulty.
3) no longer have their music cues play when spawning in (if possible).
4) will rarely, if ever, spawn in more than one at a time.

1) have a substantially reduced tongue distance (achieved through tongue_range convar).
2) increased tongue fly speed (achieved through tongue_fly_speed convar).
3) no more smoker ai tongue firing delay (achieved through smoker_tongue_delay convar).
4) no more smoker tongue miss cooldown (achieved through tongue_miss_delay convar).
5) have considerably reduced health, and are only about thrice as durable as a common infected (achieved through z_gas_health convar).
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This results in them needing to close much more distance and be within 10-15 feet of a survivor in order to use their ability, prompting a survivor to pick a corner and hold it careful if in close quarters, or try to move out into the open where the smoker is the most vulnerable. As Overrun discourages going out in the open, a smoker pushing a survivor out into the open as the safest tactic available would be a fun switch-up.

1) will never pounce and instead run silently toward targets to scratch them
2) do not turn and leap away after taking damage (achieved through hunter_committed_attack_range convar).
3) have considerably reduced health, and are only about thrice as durable as a common infected (achieved through z_hunter_health convar).
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This results in hunters functioning as dead-silent auto-aggro'd common infected with boosted health, giving them the potential to jumpscare a survivor in close quarters who was patiently checking every room, and thus naturally teaching a survivor to always close doors behind them so a hunter that might be on the way will be forced to make noise before reaching them and doing damage. This means hunters, like smokers, are most dangerous in close quarters, shaking up and challenging the sense of absolute safety that close quarters usually brings in Overrun.

1) will continually leap toward a survivor at any distance (achieveable via the z_jockey_leap_range 9000 and z_leap_interval 0 convars).
2) have slightly buffed leap power, making their leaps take them farther with each one (achieved through z_leap_power 500 convar).
3) have moderately reduced health, and are only about 4x as durable as a common infected (achieved through z_jockey_health convar).
4) will not use scratch melee on survivors/does heavily reduced scratch damage to survivors, but will still use it to break down doors or breakable objects in the map.
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This results in jockeys essentially being harder to hit due to constantly leaping off the ground over and over, as well as closing distance to a spotted survivor faster than any other special infected. This prompts the survivor to spray and pray in a panic and backpedal at full speed to buy themselves more firing time once the jockey is spotted, making the most effective tactic to deal with jockeys cause the survivor to break out of stealth and fire their weapon wildly. Such a break of stealth can attract many commons if out in the open, or prompt the survivor to hide in a room as they hear the jockey approaching so as to avoid an instant ride-grapple.

1) have substantially increased health and can tank roughly half a magazine of bullets from tier-2 rifles
2) have no vomit delay after acquiring a target (achieved through boomer_vomit_delay convar).
3) will immediately seek cover if spotted at a distance (achieved through boomer_exposed_time_tolerance convar).
4) move slightly faster than normal.
5) can vomit repeatedly with no cooldown (achieved with z_vomit_interval convar).
6) recover from a stumble faster than the survivor's bash cooldown, meaning spam-bashing will not prevent the boomer from vomiting unless you 'juggle' bash him with a friend in co-op.
7) will not use scratch melee on survivors/does heavily reduced scratch damage to survivors, but will still use it to break down doors or breakable objects in the map.
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This results in boomers not only having the capacity to summon all the large quantities of idling commons spawned in in the survivor's near region, but since being vomited on shakes your view violently and can ruin your aim while a boomer will repeatedly vomit on you until dead or you move away from/bash it, it can seriously impact your ability to not only deal with the massive horde now closing in, but also your ability to retreat if you're in close quarters so long as the boomer is still alive, forcing a survivor to focus the boomer first and the horde second, wasting valuable retreat time.
This makes boomers extremely effective ambush specials in close quarters that do no direct damage to you, effectively being able to lock you in place until they are killed, making them exceptionally dangerous by maximizing the power of horde summoning, which makes sense since the main gimmick of Overrun is enormous zombie count.

1) have no cooldown for their spit ability (achieved through z_spit_interval convar).
2) move moderately faster than normal (achieved through z_spitter_speed convar).
3) can spit from substantially farther away (achieved through z_spitter_range convar).
4) have faster flying spit that can travel as far as the spitter is able to target a survivor before the spitball drops to the ground (achieved through z_spit_velocity convar).
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This results in spitters essentially acting as long-range area-of-denial that will fulfill their vanilla purpose of flushing a survivor out of a corner or chokepoint but with much greater efficiency and from much farther away. Spitters will now spit anywhere from once to as much as 4 times in a row before retreating for cover so long as line of sight with a survivor goes unbroken. Combine this with their vastly buffed range and faster movement speed and they are virtually guaranteed to always get the first shot off before dying unless the survivor uses a sniper. Overrun spitters shine brightest out in the open where they are hard to aim at while continually pelting you with spit puddles that force you to break stealth and repeatedly move out of each one as it appears, which further stunts your ability to aim precisely at the ranges the spitter is engaging you with.

1) will never use their charge ability (achieved through z_charge_duration 0 convar).
2) have moderately increased health (achieved through z_charger_health 1000 convar).
3) scratch melee attack does 3x the damage of a common infected hit on any given difficulty.
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This results in chargers functioning as miniature tanks that are designed to push a survivor out of an enclosed space because the survivor needs more running distance to stay away from the charger OR push the survivor deeper into close quarters and force them into a room and shut the door so as to have a door between them and the charger upon its arrival. Similar to jockeys but with no auto-capture ability, chargers being spotted/heard will prompt a survivor to break stealth and backpedal to avoid damage, as chargers cannot be bashed like jockeys. Should a charger manage to close all distance with a survivor, then it'll be capable of downing them in only a small handful of hits even on <expert difficulties.
Chargers will thus become the most important special infected to prepare for via equipment choice, and will drastically bolster the value of explosive ammo packs which can cause a charger to stumble with each shot, buying precious time to create distance and do more damage to him.
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Part 1/2, see first comment below.