I think the BEST approach would be to use the Axiom full-metal ceiling bracket, which I believe will allow you almost infinite adjustment between the angle of the ceiling (or the sloped walls, in this case) and the speaker.
+1.
I can almost guarantee that FMB's will be Axiom's suggestion. The QS8's are pretty heavy speakers. They need to be securely fastened to the wall, whatever that wall may be. I wouldn't trust the built-in T-brackets on an angle. Just too much to chance. Really, just email Axiom. They will know. The great Alan himself will probably respond within a day or so.
There's a good thread about marital-bliss and home theater in the Water Cooler area...
http://www.axiomaudio.com/boards/ubbthreads.php?ubb=showflat&Number=252047#Post252047
