Well folks, last week I had the opportunity to test my M3 against a well driven C63 AMG in the rain at Summit Point main.
With a worse driver (me), on completely worn down stock tires (I mean gone), I can say with certainty the M3 is about 0.5 - 1 sec faster around a 1m30 lap. (Well it's probably close to 1m25 in the dry, who knows in the wet).
It's hard to say exactly how much faster, but at the rate I was approaching it could have actually been more than 1 sec. Unfortunately the session ended with a car off before I could press the advantage.
Just to dispel any potential driver arguments, it was my first time in the advanced group and the other driver (a neighbor it turns out) is a lot more experienced than me (drives a 911 cup car too).
So I can definitely say that the M3 is faster. And since my exhaust installation, sounds better too.
Waiting for the excuses now in 3, 2, 1...