I assume enemies give less XP in general since for me it cost thousands to get levels towards the cap. But apparently the cap on the Turbo is really high (if there even is one) so if you really want to you could grind to the point of one-shotting bosses.
Most enemies early on give 2 or 3 XP per kill, then scale down as you level up to 1 XP per kill. I've not noticed what kind of XP that I'm getting from enemies in the tower.
I just looked. At L21, the first creature that I found on L9 gave me 12 XP per kill. I currently have 3355 XP and need to get to 3500XP to hit the next level.
Should I have the silver armor before attacking the boss on L8? Or am I current on my gear at this point?
EDIT: I did some grinding in L9 until I was L25. At that level, the mobs that gave 12XP per kill started to give me 3 XP per kill, with the next level needing 400XP to get, so leveling started getting slower. I then tried to beat the L8 boss, and it was easier. My hits started doing more damage (probably closer to my 5% estimate vs. the 2% that I was getting at L21), and I could take more hits (maybe 8-10 hits instead of just 3).
I guess I'm just used to how games like Final Fantasy and Dragon Quest scale, where there is little difference between levels, but in a game like this, 2 levels can be a significant difference.