The clutch spring mod is covered in the thread below and the rennlist one linked in TRK's first post. Short story is the McMaster spring recommended toward the end of the rennlist thread is what I did, and I love it. Mailed out my spares (had to order min of 6), so we should have some more feedback on them soon.
http://986forum.com/forums/performance-technical-chat/55610-clutch-mods.html