Why not move your QS8s to the rear first and listen to some material in the position you're thinking about?
I'm using QS8s for my side surrounds, and the only thing preventing from channel 6 and 7 is not physically having anywhere to put them in my current place. Once I get moved and building out a new area, my pick would still be adding two more QS8s (unless they introduce a better one i'll want four of.. they tend to do this!)
IMO, it's less to worry about. They're a better match for more sitting positions (since they emit in orbs instead of lines) and I've been really happy with them in 5.1.
(when you get to 7.1, check out the final action scene in 3:10 to Yuma)
