Actually, a trackday is the best time to turn off the TCS, especially if it's raining. As long as you're mindful of other cars and not driving like a complete tit, you can push it and spin out in pretty much complete safety: After all, you could never do that on the roads and how else are you going to get a feeling of what to do when everything goes wrong? Whenever I get a new car on a trackday the one of the first things I try and do is overcook it and spin on purpose, just to see how the car feels and reacts.