1. the extra damage is op for minisentries
2. xray is op
3. less damage for dispensers and teleporters isn't much of a drawback
4. you don't usually go invisible with sapper anyway
5. -10hp what if you already have kunai?

+Damage increases as it sapps a building (up to +20% more damage to buildings)
+200% meele resistance (6 hits with wrench to remove, 9 with Jag )
-100% damage to buildings
-On hit damage increasing resets (the damage increasing resets back)
-Cannot sap dispensers and teleporters

This will make the sapper once it's placed a fight against the engineer instead of the classic
*sap* *sap removed* *sap* *sap removed* *sap* *sap removed* *sap* *sap removed*
building gets destroyed and then it's a fight between the engineer and spy

With the sapper once it's placed there will be a fight between an engineer and spy. this is useful since it diseables the sentry and lets your team attack the sentry or let's you fight the engineer at the cost of not damaging the sentry until it has been placed for a couple of seconds

all you say is nice. but let me do a small change :
+Damage increases as it sapps a building (up to +20% more damage to buildings)
+200% meele resistance (6 hits with wrench to remove, 9 with Jag )
-100% damage to buildings
-On hit damage increasing resets (the damage increasing resets back)
-10% hp on wearer.

yes i brought back the -10% hp.
but it's a risk you must take with this sapper.
i removed your "cannot sap dispensers and teleporters" because it's not a sentry-locked-sapper.
sapping dispesers and teleporters is sometimes better then sapping the sentry.

Bi-Decoder
75% sapper health
50% sapper damage
On MVM: damage's the robots
Can only use one sapper at a time
After your sapper is destroyed/used, it needs 10 seconds to recharge