I'm not really comfortable with the way the races work, especially the shorter ones, as it's way too easy to DQ. My FTP is around 300-315, and I know I can hold that range for about an hour, it's really really hard, and I don't like it, but I can just about do it.
For 20 mins though I can hold significantly over that, especially if I'm pushing to say 500W and then getting a recovery time around FTP or just below, this means it's really easy for me to DQ from say CAT C even though nominally I might be within the category based on my FTP. Now you could argue that I should be Cat B, and I'd probably agree with you, but my point is that just because you can push well over your FTP on a short race doesn't mean you should necessarily be disqualified.
A better system might be a points based ranking system like British Cycling use. Everyone starts in Cat D, and works there way up based on results, or not.