Bob a friend of mine just sold a racing TR8 he developed over 5 years.
He had 14 inchby 6 alloys, 205/50 Kumho Victoracers; obviously wider rims and tyres would have been better but not legal to stay in class.
For autocross you need tyres that heat up quickly and pads that will work at moderate temperatures, not the same as a 15 lap race.
Simon.