Haven't had a Trek in a while, but isn't essentially a 4-bar system?
I test rode a EX 9 (or 9.5.. or something?) a while ago at it was pretty efficient. Not xc race efficient, but definitely had a stable enough platform not to slow you down much.
Also, I wouldnt worry about a carbon MTB. Yeah, we've all heard horror stories, but if it didn't work they wouldn't make them. The rear triangle of my FS last year was carbon and was hit by it's fair share of big ol' rocks and is still strong as ever.