Paul, other than the T5 that I broke in my last Cobra, I also don't have much personal experience with high perf manual trans cars. The factory T5s over here were the plain Jane versions. I honestly don't know how to tell the difference between the two without lining them up side by side. I can tell you a bit about 1000+hp capable Powerglides though....
David was/is using a McLeod Dual Disk Street Twin clutch assembly. Bit of a mouthful, but apparently a really strong setup. For a lot less money, you could get someone like Denis at Direct Clutch up here in Brisbane to build you something light and bulletproof.
http://www.carscene.com.au/outlets/direct/index.htm