Hi Mike, welcome! Andrew is right; some people are much more likely to click than to cut/paste, so he was just trying to get those folks to your link.
Unfortunately, I too can only offer opinion and anecdotes, not actual experience. I run 5.1 in a pretty big room and really don't feel deprived. In looking at your space, I should think that 6.1 would be quite adequate since it is not a terribly wide room. Plus, the mounting for 7.1 would be pretty challenging. One of our board regulars (Spiffnme) went from 5.1 to 7.1 and ultimately decided that the improvements were too modest to justify the expense and hassle. There is (to my knowledge), no discrete 6.1 or 7.1 source material anyway, so at best, your getting matrixed or derived channel(s) back there anyway. People seem to like the HK and Yamaha surround chips for those kinds of installations.
There have been discussions here in the past about which speaker to use for the rear channel. There does not seem to be a clear consensus, with some people preferring a third QS model and some preferring a direct radiating speaker (like a single M2). Getting a professional opinion from the Axiom staff might be worth your time.
As far as multichannel audio goes, the QS8's are terrific speakers. If you are concerned about their lower extension, you could consider an Outlaw ICBM or get a receiver that allows you to adjust the sub crossover for each channel independently. You don't say what kind of equipment you have or are considering, but bass management capabilities seem to vary pretty widely on DVDA/SACD players. The QS8's are really very capable speakers; the drivers are credibly big as is the enclosure. Unless multichannel audio is your primary focus, the immersive surround offered by the quadpolar design is a great HT/Music combination. If multichannel audio is going to outweigh your HT usage by like 70/30 or something, you might want to consider getting 6 identical speakers all the way around.
If I were in your situation, I would do 6.1 with 3 QS8's and be totally content.
