Whether you have 5.1 or 7.1, the left/right surrounds should be to the left/right of the listening position either slightly in front/behind a parrallel line to your ears.
You need some seperation of the left/right channel and it does not sound like you will get this from behind you and above, as your ears will be aimed to the right/left.
I would think the sides, even if they are close to the rear corners will work fine. The rear tweeter would bounce off the rear wall and give you a simulated 7.1 experience in a way.
I just don't think 10ft up, behind you would give you the same experience.
