Hard to prevent cheating - someone can take off in the category ahead of them. Unless you RFID the starters or something, who'd know? Also, faster riders from the next category will catch the slowest riders quite quickly. This is a frustration the fastest riders don't current have an issue with. The standard mass start typically means the front 30% of the field has no congestion issues.
I think it's best to have a good long run and couple of kilometres of firetrail - preferably with lots of corners and a fair bit of climbing - to sort it out.