I did a Cat5 crit last year. Average speed was about 22mph. I stayed on until the second last lap when there was a crash. Someone hit my rear wheel (I stayed upright but lost my nerve and the pack).
The guy who won went off the front on the first lap and averaged a hair over 24mph. He upgraded to Cat 4 quickly, and from having ridden around him occasionally should be at least Cat 3 I reckon.
Toughest things were for me, as expected, accelerations out of turns, and if you're not near the front, the fact you'll end up having to slow down downhill and accelerate uphill.
Last edited by idc; 03-01-13 at 08:04 PM.