1. The C63
does not weigh that much, from what I recall, it's more 1.8 tons at most. You're thinking of the SLs.

2. I'm guessing that "tuned" means "Add power, minus weight" in this context.
Anyway, @OP, think of it this way. The M3 is a track car, balanced handling and all that. The C63 is a C Class with a socking huge V8 shoehorned under the bonnet. It's more about POWERRRR and having a lot of fun going slideways than pure lap times.
C63- Gentlemanly hooligans who like a Merc and enjoy the immense power. (Is that actually a type of person
?)
M3- Track enthusiasts who like a BMW and enjoy
er
the track, I suppose.