NEB and Bob Newby Racing both come to mind, I'm sure there are others.
The Phil Pearson one was / is (?) suzuki based.
Or likewise there is the SRM 4 spring pressure plate (to improve the lift method -has a top hat bearing arrangement I recall)
As soon as you start deviating from stock the next question is belt or chain driven.
My locally modified suzuki clutch is still chain drive but I really love my BNR belt drive on the race bike.
I do still run a 4 spring on the 500 single and it works, wouldnt be my first choice but it works.