If the rear speakers are ceiling mounted, couldn't you move the the LR closer to match the distance of the RR? If the tweeters are rotatable, you might want to try (emphasis on try) actually pointing them away from you and having the sound reflect off the back wall. You can experiment with angles to compensate for the angled wall. FWIW, I personally wouldn't even bother. I'd stick with two channel. IMHO poorly executed surround sound is worse than well executed stereo.