The up-side-down part is cool, but if you do this make sure that you don't restrict them to being upside down for too long unless you are planning on making that part REALLY hard (like an insane mode) unless you will perhaps be allowing it so you can go all the way around like in a spiral or something.

Doing a whole race up-side-down would be very hard. But it can work to add some sort of a *different* kind of racing challenge.