I'll give u my take on it....
I don't know if FCP will benefit because its still the same physical disk so it wouldn't really perform better
I have a 40GB drive and I have three partitions, 10GB for Jaguar, 10GB for Panther, and the rest for Junk
For a while I was messing with panther betas so it was nice to be able to jsut format that part and when I get retial panther I can just install it and go ... a fresh install and no worries if i need to go back to jag for something. Simialry if panther is werid or funky, I can goto back to jag with all my stuff intact and still quite useable.
I've had to reinstall jag when I had my iBook several times (that was apples first suggestion for my logic board torubles ). They suggested an erase and installl... if i had kept my documents, downloads or movies or mp3s i'd be screwed with an erase and install... I like the security of having that stuff on a different partition so if your OS screwes up the stuffu really care that can't be reinstalled is still there. The same efect can be produced by regualr good backing up but thats less convenient than just paritioning to me.
thats what I would do, also remember its easier to partition and reinstall everytihng when the comp is new than after you've got everything loaded up so if your going ot do it make up your mind fast because you will have to erase the wohle drive and reinstall.