The floor seems to be pretty close to level using a 3' level. I am sure there is a slight slope. It is about 8' below the main floor. I don't mind it remaining below the rest of the house. I am more concerned with making the floor level, although if it's a slight slope I might leave it as is. I was thinking the wiring would go over the top of the room since I would not use the garage ceiling for the room. I want to leave room for the garage door to go up. I just measured and the lowest point with the garage door up is 7', with the garage door down it's about 8'. Assuming a minimum of 3 or 4" of clearance, that leaves conservatively 6'8" for the ceiling. I don't know if that will be enough. I leave the garage door locked down, remove the door opener, I have 8'7" and can use the garage ceiling. There is a power outlet in the middle of the ceiling that I may be able to use. This would make the job easier and I can keep my access to the attic which is in the garage for that side of the house.
Interesting idea but I will probably use the room for a home theater so I would want to have the as much room as possible for seating. I also want to keep it as simple as possible. I've seen some rooms done in the manner you have suggested and the must sound great but considering my listening area at the moment, I'm willing to sacrifice some room acoustics for added space.