I'm a little dubious about using the T-bracket. It's probably just fine, but it is designed for the force to be fully vertical. Using it for an angled installation might impose forces it is not designed to tolerate.

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.

bibere usque ad hilaritatem