Have you found your brakes to be inadequate, or are you just wanting something better?
My 300E brakes have been incredible, and I have done some high spirited driving in the past.
I think that your best course of action is to measure the rotors to see that they are within spec. and replace the pads with good quality pads, and enjoy your car.
In Autocross, you don't go into enough corners in the course of a run to build up much heat. If you were road racing lap after lap, I could see where this would be a concern.
If you have driven the car sufficiently hard that you have found a shortcoming in the brakes, I'm anxious to hear about it, and your ultimate solution.
Good luck and Merry Christmas,