I coasted from a dead stop to 38.5 mph on a quarter mile of 8-10% descent this morning. I've gone as fast as 45mph coasting on a longer descent before I got nervous and started braking. Of course I weigh a bit more than your average TdF rider (and by 'a bit' I mean 50-60 pounds), so gravity helps me more.

It's true that they won't be able to move the bike much faster than 50 just by pedalling, but if they pedal up to 50 mph and still have some slope in front of them it's all a matter of gravity, aerodynamics and terminal velocity. I don't see why 60 mph wouldn't be reachable.