I have an HK 235. My side surround speakers are on the side walls and I have direct radiating speakers on the back walls. Even with QS8's on the side, I find myself changing everything to 7.1 because the rear speakers add so much depth and realism to the surround effect.

If one's surrounds are behind the listener (mine are actually about 1 to 2 feet in front of the main listening position), I don't think the 7.1 difference would be as noticable. I love the way 7.1 sounds, even with media encoded for 5.1 formats, but I think every setup will be a little different.