FYI, I have a 7.1 QS8 v2 installation. My back surrounds are ~3-4 feet behind my listening position. I find the surround field fantastic and makes my back wall disappear (or at least sound way further back than it is). I have not listened to the QS8's further back than that.
On-Wall M5HP LCR, QS8 & EP500 in 7.1