Sounds (pun intended) good to me.
5.1 will do you very well. No need to go to anything more than that. I just went up to 7.2 (construction in progress), but that was because my room is 24 feet deep and I needed more surround coverage for 2 rows of seats. I doubt that I will notice anything spectacular in the 2 extra channels for movies encoded with 7.1, but having the area enveloped will be key.
Your space will be more than fine with the pair of QS8s. My most recent theater (before this one) was 15 feet deep, and two QS8s worked great.
