I don't autocross, but I believe conventional wisdom is that the thermostat limits how cool the car gets, and the radiator and fans determine how warm the car gets. If you need to stay cool, focus on the radiator and/or fans. I have run larger than stock radiators on both of my cars and on my wife's current car, and the only cooling problems I have regardless of load, outside air temp, or traffic is when there is a leak in the cooling system.
Others can probably give better advice. If you need to stay stock, the radiator and/or fan mods are probably out of the question, and the advice above is probably useless.