If you look at the fastest speed thread you get a lot of people around topping out between 45 and 55, with some up into the 60s. I would assume most are doing it on a standard set up.

I would guess most of the tour riders are doing it on a double that is 53/39 and have a cassette that might be 25/11. Remember these guys go up mountains at over 10 mph. I figure i could handle about 2 mph on those, as I pushed my bike up.