#1
Does playing without the back panel of an amp affect the playing?
For good or worse (exp. good=louder or bad=hurts amp)

I'd heard it makes it louder, which'd be nice considering I've only got a crappy 10 watt ATM
#4
Certain amps are made with an open back design in mind. And Vice Versa.

Any volume boost would be near unnoticeable at 10 watts.

I say leave it alone.
American Strat:
Area 61 Neck, Area 67 Middle & Bridge
Spretzel Locking Tuners
Graphtech Nut & String Tree
Lots O' Love!

JSX + Orange Cab
Fender Blues Deluxe
Deluxe Memory Man
Fulltone Full Drive 2 & OCD
H2O Chorus/Echo