Two observations.
1 if you think those guys are fast you should see the Euro pro's go at it they make our domestic pro racing look like a club ride.
2 The difference from a cat 5 to a cat 4 isn't that big of a leap. C4's just go longer not much faster. The jump from 4 to 3 is a bigger jump longer and faster. 3 to 2 is even bigger but 2 to 1 isn't that big of leap. The gap from 5 to one might as well be from here to the sun. It's sick how fast these guys are and that they can ride that fast for so long.