Lex - let me jump in here and give you my impressions.
I have a dedicated basement HT with QS8 surrounds about 3 feet above ear height, exactly to the left and right of the main listening position.
I enjoyed this configuration for about 2 years before upgrading my receiver and installing 2 rear channels (direct firing - not the QS8) in the rear. They are about 4 feet behind the listening area and hung from the ceiling.
My impressions: The QS8s alone filled the room with sound and I noticed no gaps in the rear soundfield even during pans. The reflections and dispersion characteristics of the QS8 really provide a nicely diffuse rear surround environment.
After installing the 6th and 7th channels behind the listening area, the sound field filled in slightly, but the difference was not so noticeable that I said "oh, wow, I've been missing so much." The two rear speakers do help with pans and overall filling out the rear sound field. But as far as getting direct spacial cues, the QS8s fill that part of the room so well with sound that the cues are often undetectable (a good thing for ambient effects).
My recommendation would be to invest in two M2s and wire them in stereo (in a 7.1) configuration. This will future-proof the setup a bit. The 7.1 will give you a stereo rear surround and will also permit you to tweak the separation of the rear speakers to optimize the rear sound field, as opposed to hanging a single QS8, with really only one option of placing it directly behind the main listening position.
