I have also done an instal with Meridian 5500's as Center, sides and rears, mounted in a soffit and angled to the listening spot of choice. With the boundary correction software thru the Meridian 861, it sounded amazingly good, BUT the center higher than the floor 6000's, was a bit odd in L-R pans, for example. So, if you are thinking at all about using floor placed L&R's with that height differential-be very careful. The soffit,in that case was contiguous with the attic space and in So. Cal., and no problem with temp changes-YMMV. Without doubt, the all floor Meridian suite I did with 8000's all around was superior., in most regards, but I did not prefer the floor placed rears.
Now, the Sonance product I mentioned, with pivotable tweeter, made the sound stage psycho-acoustically drop, albeit with a totally different net effect. Sounds like your room is an acoustic nightmare, rife for all sorts of acoustic treatments that you can't employ. If there ever was a case to listen to the ceiling application of some of these options, this is it. Has Jim ever created a speaker with a pivotable tweeter, etc.. Could you build a soffit and house the speakers in it?
Thank you very much
Fury