FWIW I've tried 16mm,17mm,18mm, and 19mm m/c's with my M50s and I prefer the 19mm by far. It all depends on how you like your brakes to respond but I'm certainly happy using 19mm and, contrary to what some say, it's not dangerous or wooden as I have loads of feel and modulation.
Cracking price that. Shame I popped down to London this very morning to get some M4s or I’d have snapped your hand off. GLWS
Well, the hpk's I got for a brilliant price and they went on my zed (the z calipers were the oem no name spec) so, there was a marked improvement in performance plus, my mate held on to them so I could pay for them bit by bit (I could never afford them otherwise)