I take a different view.
Not everyone has the background that magically produces a good mechanic and I doubt if there are many good mechanics out there that haven't cocked up many times on the path to competence and crucially have learned from the experience. let he who is without sin cast the first stone and all that...
Jonathan has made a mistake, admitted it and is learning the hard way just how expensive it can be to make an error and not catch it before it matters.
I've never had brake fade in my M3W, can lock the brakes at any speed at will on any dry surface should I choose to so there isn't a great deal wrong with the brakes overall. While I don't circuit race my car, I suspect that would be the only case where brake fade in a correctly maintained M3W could be an issue. Certainly in the track days I've done I've had no problems. Last time I was at Caramulo I did about ten consecutive high speed untimed "parade" runs up the hill in quick succession on the Friday before the competition again without issue. Even on the quickest of my "it's not big and it's not clever" early morning laps of the TT circuit last week I had no issue despite some of the high speed slow downs required which is more than I can say of some of the road bikes I've ridden round there! Hence my thoughts that the disk conversion is unnecessary, however nice it may look. I also happen to like drum handbrakes once they are set up correctly they have a tendency for excellent engineering reasons to be very effective and rather better than most disk systems are.
Well said, Callum, and great to meet you in person finally on the Isle of Man.