Sure, that may be true. Or they may rate it low so you buy the higher end clutches, or maybe they are just conservative ratings.
I had an exedy stage 1 in my old 240sx with a skyline engine. It was only supposed to hold 255. The stock clutch should have held much more than that. I was pretty abusive and it never showed signs of failing.
It also depends how well the f30 hooks up on the 255 tires and the e-diff.