There's a difference between the warranty and the maintenance plan. Just to be clear. Rotors, brakes and the like are covered under the maintenance plan. These items will wore quicker if the car is tracked. Also I believe at some point MINI had guidelines about what's covered and how often. (Like oil changes only every X number of miles but you can pay for a more frequent interval.) The warranty covers the motor, power steering pump, etc. Tracking your car shouldn't kill true warranty items. I tracked my car and my local dealership still replaced the brake pads and rotors no questions asked. However if you as serious about tracking as you've indicated elsewhere, upgrade your tires, brake pads and rotors. The least one you want on the track is brake fade. Just my two cents.
When I got my 2006, I kidded the dealer about the brake pad wear I would experience. He said bring it on. And I did. And they covered, covered, covered. Tho the rotors were seldom worn out, if they replaced the pads, they replaced the rotors and sensors. This dealer owns two Mini locations and a BMW dealership. He sponsors HPDE events for customers. He supports two Mini Clubs and the local CCA chapter. Good guy.
You really just need to stop worrying and do it. Some of the most fun you'll have with your clothes on in your life.