There are arguments for all ends of the spectrum. Some folks swear by the 20 pound stocker, saying anything lighter than that and its hard to launch, RPMs fall to quickly, etc... when compared to those 7.5-8 pounds jobbies.
Then again, I've seen vids of some 13 psi cars running the ultralights without any problem and they swear by them. I don't know. I split the middle and went for an 11 pound Exedy. I'm not boosted (yet), but I love my new setup. IMO it offers the best compromise between reducing my rotating mass for a quicker rev, and keeping the load for a faster spool (when I get boost). So there's my .02.