Mr. Quacky (yes Randy, that is you) is right. It is sort of like the opposite of when a receiver takes a 5.1 soundtrack and "mixes in" its own 2 additional channels to output sound for a 7.1 system. In this case, it just "mixes" a 6th primary channel instead of a 6th and 7th for 5.1 content.
For native 7.1 content, the receiver would just change the mix to sort of combine the 6th and 7ths channel into one.
Ideal for the best sound reproduction? No as you are still combining 7.1 sound into a 6.1 setup, but then again, we aren't dealing with an ideal situation.
On the counter-point, I think that you could go with two QS8s mounted as "wide" as possible. It may end up sounding just a little better than a single QS8 mounted in the middle, but the cost trade off will need to be made. Is it worth the cost of 2 rear speakers if you get the effective sound of (I am making this number up) 1.4 channels (as in more than 1, but less than a solid 2)...
Maybe it is worth it.
