New to alchemy in general, apologies if I say something incorrect.
I used a Cursed Infusion on a +5 Bow (might have been +4?) and it didn’t upgrade it at all.
After drinking 4 Potions of Experience to get to Level 30, the Bow only levelled up to +6 and stayed there after I removed the curse.
I’ve seen people get to +8 so I must have done something wrong.
It is based on the character level, up to +5. I believe, the only way to (effectively) upgrade the bow is by using a ring of sharpshooting