I ran across a chap who worked for Prodrive for @ 15 years and based on their testing they use Motul RBF 600 for the brakes and Castrol SRF for clutches. Not to just play copy cat, but if Prodrive did the research for those selections I'll take heed.

For racing use the wet boiling point shouldn't be a huge factor (negating the need for the liquid gold Castrol SRF). Some of the cheaper hi-temp fluids are available from Wilwood (EXP 600) and Brembo (LCF 600). Motul RBF 600 is only slightly more expensive. All are cheaper by the case, which is a reasonable consumption amount for a full race season.