I raced on a MC28 with a rear rotor that I'd machined down to 3.5mm. Now that's a healthy weight saving if ever there was one!

I raced on it for two years with not one problem.

I'm not a great rear braker myself so a skinny rotor wasn't an issue.

If it was my mate Shane on the other hand, he can fry an R1 Braking rear rotor in just a handful of laps. He's a big lad and must stand on the bugger 90% of the lap!