How far will the back row be off the rear wall?
I had a great 7.1 (actually 2) set up in a room 19' deep.
If your seat is in the front row go 7.1 with the L&R surrounds at 90° (I prefer 87.5° with QS8 surrounds).
For what its worth I'm considering placing my QS4's as my L&R surrounds because my new room is 11 feet deeper. The 4's are good on their own own and I'd like a bit more from my rear surrounds.
"No one listener, therefore, can be the arbiter of absolute quality."