I know clamping force is what stops the slipping; hell, I even considered just shimming the diaphram spring with washers for more preload! - I just assumed the "highest" performance clutch there would have the best clamping force.

So does an A15 clutch fit a late model A12 flywheel and B310 gearbox? If so then a HD A15 clutch is the go and should be cheaper yet.

Also wouldnt a button clutch slip more as there is effectivly less surface area in contact between the pressure plate and flywheel?

I reckon a full face HD organic plate and a decent pressure plate would be the better setup for me (not an all out race car but is does make a fair bit of torque).