#1
In terms of sound quality ? I mostly play with some crappy audio technica headphones (m20x). I have an epiphone sg special and a g400 pro. Also a Takamine EF341sc which should be good with the Katana's acoustic channel. I'll probably be using the Katana on the 0.5w option with headphones most of the time. 

Buying a g400 pro over the sg special was a huge increase in quality for me, I find it hard to believe it can get even better.
#2
Side grade? Especially if you are playing over headphones.
Prs se Holcomb is the answer