I like the idea, also when someone is knocked out, you can force them into /stunt or /tt. It might get boring and the race might be too long so I see a lot of people leaving if they get knocked out. One way of keeping it short is make it so the last person to reach the next CP is knocked out, but make sure to space out the CP's evenly and make sure they aren't near each other. This should be TT by default I think, and the knock out stage should start at CP 5 for example to give every one a good running start.