I am going to be putting my QS8's on the rear wall much like you are talking, but I have the wires run so that they will not be directly over the couch, but wider.
When I asked Alan from Axiom about rear mounting my QS8's, he said that he has his couch on the back wall with the QS8's mounted on the back wall as well, so it must work fairly well.
I am mounting mine about 8-10 inches from the ceiling to allow sound to reflect better off of the ceiling to create a better sound field. To close to the ceiling will "trap" the sound, and too far away will have them too close to ear level.
I think that you are quite a distance away from your 51" TV too, but hey, it is your preference, not mine or anyone elses, that is what is important. (I am looking at a 96"-106" screen from 11' away. I've tested the 96" screen with an old projector, and I thought that I could easily enjoy something even a little bit bigger.)
Best of luck...
