I love them, frankly.

In the demo, I was kind of worried how you would balance out collecting so many damage upgrades. I never even considered limiting their effects to their respective level! It's such a cool little incentive to explore a level and find its secrets - by changing how your weapons interact with certain enemies.

That said, I would like to put a bug in your ear about them.

It seems like the most significant changes in kill threshold come from collecting all 3 violins. I think this undersells the system a bit. You're only going to be expecting an all-violins run in most levels from completionists and the most diligent of players. Most other players won't find them all in their first playthrough, and if they go back for them, it's not gonna be for that 30% bonus. They'll have already conquered the game.

I don't think that this aspect of the game gets to shine like I hoped it would. So I wanted to pose an adjustment:

1st Violin - +15% damage
2nd Violin - +30% damage
3rd Violin - +10% fire rate

I was able to find 2 violins on most levels in my first run, and I figure most other players would hit that mark too. Now they can reap the full benefits of Violin damage bonus more often. The 3rd Violin offers a smaller extra bonus that rewards thorough secret hunting, but not so much that you'd feel compelled to go out of your way for it.

On top of that, maybe an extra 5% on that 1st violin might change some other kill thresholds in interesting ways.

Alternatively, you could also not TELL the players how much their damage goes up (just put something like "DAMAGE +" there instead), and manipulate each weapon's damage under the hood, to have more control over when they cross certain thresholds.