I guess this will work as well, but orthodox placement for the "side surrounds" is on the sides, not in the back. I did not experiment much with having only the backs - I always use the sides. I find that even in the 7.1 setup amount of information coming from the side is much more than from the back. For the music it is the sides that give the "aireness" to the 5.1 surround recordings (SACD/DVD-A) while the backs merely improve it incrementally. The movies also use the sides much more often. Having any real content in the back channels (EX/ES) is rare. I suggest you try both - with some temporary support stands - and see if you hear the difference, only then start drilling the holes. In my case positioning of the side QS8's was way off in the beginning as well - I put them too much forward and experimenting with both the height and the distance from the back wall helped in the end achieve the sound I wanted.
