My room is 20.5' deep, and 14' wide. The height from the unfinished concrete floor to the bottom of the joists above was exactly 8 feet. However, from that, you need to subtract 12" for ductwork that ran down one side of the theater (which I framed all the way around the room for symmetry), 7/8" for Dricore panels, and 1.25" for double drywall. The rear platform is 13" high, so standing completely upright in the rear row is a bit tricky. I would have loved another foot of height to the room, but it wasn't an option.
In any case, putting sconces on the speaker columns would have presented another issue: Protrusion into the seating area. The front two columns (which have sconces) are not in a walking or seating area, so it's OK that they protrude into the room. However, because I put 4 seats in my front row, and because you have to walk right by a column to get to the front row of seats, putting a sconce on the face of the column would have required people to turn sideways just to walk down the side aisle. Since I had plenty of can lighting, I really didn't need the column sconces for illumination (they are only decorative), so eliminating the sconces from the surround speaker columns wasn't an issue for me. I've seen some theaters that rely on the sconces to actually light up the room, but my main lighting comes from overhead.
jj