I have a similar layout in my home. (open greatroom, vaulted ceilings). The only thought coming to my mind is light. Don't know if you've checked, but shades for trap windows are mighty expensive. I have traps throughout my house and no shades because of the costs.
I experimented with my speakers on the fireplace hearth which is 16" high. I preferred them higher but they just didn't look right. I have rock and a hand carved mantel surrounding my fireplace and the speakers took away from them. I spend more time standing in the great room / kitchen area than sitting though. But also liked them higher when I was sitting down as well. I'd just experiment if I was you.
Oh, and one other thought......does you wife know your plans? When I had one of those, she refused to let me have a TV in the greatroom.
And you don't 'need' the M80's. Anyone who suggests them for reasons of space and volume anywhere other than a gymnasiam has not heard them.
