If it's for track only, I'd recommend either the Pirelli Supercorsa, Supercorsa Pro, the Metzeler Racetec, or the Michelin Power Race. The lesser tyres from either make will be good enough for a trackday, but these are the top of the heap short of slicks. I race with Derby-Phoenix MCC where slicks are not allowed and the vast, vast majority races on one of the above. The Bridgstones are not quite in the same league based on club racing experience.
Thunderhill is a good track, been around it a few times myself. Keep the shiny side up.