i too have 4 QS8v3 surround speakers. two are for side and two are for back; all of them work incredibly well.
the sides are amazing but the backs, they serve 2 purposes. they act as true back channels for 6 & 7 channel movies but they matrix for 5 channel movies. i have big groups over up to 12 at a time for movie nights and when watching a movie that's 5 channels everyone gets the same surround effects because for the folks sitting in the back, the sound is matrixed from the side to the rear so everyone gets their surround effects and nobody misses out!
it even works out really well for 7 channel movies. remember, the QS series projects a hugh surround field, they don't blast sound directed at you, they make you look around and try to figure out where the speaker is that made the sound!
my rears are about 13' back from the sides and it really sounds great. best of luck with whatever you decide to do.
