Paul,
My advice to you given your geometry is to make allowances for a somewhat "flexible" installation of your sides. Since your rear wall is so far back, you may have to rely on direct sound from the rear tweeter of the QSx. So you may find that you are better off locating the sides in front of your couch. Then again, maybe not.
If you decide to go down this path, place wiring slightly forward, beside and behind the couch to cover the possible side area in the diagram that you posted earlier. Or if you don't mind the wiring showing, you can just leave the wire long enough to reach all of these locations.
While you are at it, you may want to consider installing wiring for rears at your 26 foot wall in the back. Although it would have been better to have these rears located closer to the couch, your ceiling is not tall enough to accomodate the rears so you don't have much choice.
The other thing I considered was rotating your existing lay-out -90 degrees. So the TV would end up on the right 15' wall and couch 1 on the rear 37' wall. Then you can mount the rears above couch 1. The problem with this is that you'll have your ears pretty much against the rear wall and that's not the best for sound quality.
