Right now I'm running 7.5" in the front and 9.5" in the back. I came from an 8" frame and, before that, a 7" (all with the same fork). I am not sure if the extra travel is making me faster. I seemed to do a bit better at the races, but I don't know how much of that is the bike. When I first got the bike I have now, the higher BB and extra travel made the bike feel funky. I couldn't get a rhythm with it, especially on jumps. It took a month or so before it felt totally natural. I got used to it and I think I am faster overall with the bigger travel.
Of course, the "how much travel' question is pretty much not worth anything unless all else is equal between two frames....which it almost never is.
Of course, the "how much travel' question is pretty much not worth anything unless all else is equal between two frames....which it almost never is.