This isn't going to be much help to you Doc, I did fit one to my 14, and at first I thought it made a difference but now it seems as clunky as ever so I'm not sure. I'd not know for sure without removing it and I just don't have that much enthusiasm!
I do know I never paid 50 quid, more like 30. It's only about 1mm thick, I thought it would be thicker somehow. Sorry its no great help.
Having the clutch basket modified does make the gearbox sweeter, apart from 1st to 2nd.