If you're using a twin plate and its slipping (not just because it is old, I assume) just step up to a triple plate. Nearly all manufacturers of twin plates have a triple offering (eg. Nismo, HKS, OS Giken, ORC, etc.) and you can get them to feel similar to the twin plate.
BTW, I'm running an OS Giken twin plate (500hp engine) with no discernable slippage at all.