Jezza, Jim Berry clutches are basically blueprinted, increased clamp, rebuilt pressure plates supplied with the best organic disc you can get or his own manufactured button disc. And a new bearing of course. He goes to great lengths to ensure everything is 100% perfect. Zero contamination, surfaces perfectly flat, pivot ring perfectly centric with the diaphragm etc.
One of Jim's clutches won't necessarily handle more power than an over the counter clutch with the same clamp and a comparable disc however you can rely on it working exactly as it should from day one and just about guarantee it won't shudder. I've used his clutches in my own rally cars and in customer's cars when reliability and end result are more important than cost alone or when something outside the normal is required eg Tino's car.
I reluctantly used an over the counter button in my blue rally car just before Targa South West because I had run out of time to order one from Jim and it failed. With respect though it had some uphill starts with hot race tyres so it was not totally unexpected but I feel the Jim Berry button would have taken it in its stride.