There was a session at WWDC 2009 given by a couple of Intel engineers comparing the two. (Session 131, if you have access.) In their benchmark - parallel pipeli

There was a session at WWDC 2009 given by a couple of Intel engineers comparing the two. (Session 131, if you have access.) In their benchmark - parallel pipelined face detection - they were within about 1% of each other. GCD scaled up to more cores slightly better. Because it was initially built for TBB, they had to use some additional logic when moving to GCD so that frames would be rearranged into sequential order before certain stages.