Anyone tried the friction material clutch plates from Dave Renham? Any views on performance over original plates?

are they origanaly all steel plates ?

Yes, but to be honest I've never really had a problem with slip or drag.

You can generally run a lower preload on the clutch springs with the plates that have friction material, so you can achieve a lower lever pressure without any slippage. In my experience they are only a real benefit in a 325cc motor, because the 238 and 250 motors can be set up with a nice light lever pressure even with all steel plates and not slip.