I'm closely monitoring this thread, as I am contemplating the same thing and our configurations are very similar. I think the conventional wisdom is to point the rear surrounds straight ahead. Height for both surrounds and rear surrounds is supposed to be 2-3 feet above ear level, based on your seated position. Here's a link with some helpful diagrams: http://forum.ecoustics.com/bbs/messages/34579/129023.html

My only concern with going 7.1, and it looks like it may be an issue for you as well, is: Are the surrounds and rear surrounds too close to one another to optimize the 7.1 effect? Like you, I'm sort of stuck up against the back wall for the rears. It's hard to tell how far out your current surrounds are from the sectional, but it appears they would be further out than mine would be, so maybe it's not an issue with your set up.

I'd like to see some 7.1 folks out there chime in.