*) Do NOT, EVER, under ANY circumstance, even REMOTELY believe that an existing wall (whether wood or poured concrete) is even slightly straight or square. Unfortunately I made this assumption and despite measurements being right in some areas (I didn't check *all* corners, etc when building), the one side of my theater is actually about 1.5" longer than the other. You don't "see" it, but when I was hanging drywall, it was evident.
Holmes on homes discusses this pretty often in his series.. I was waiting to see if you were going to mention this as i was reading.
Nick, did you document the building of your acoustic treatments? if so, would you mind posting a DIY?