Alex,
Thread below shows the difference between 340mm and 370/380mm calipers
http://f20.1addicts.com/forums/showthread.php?t=1231761
I have just upgraded my brakes to blue 370/380 fronts and 370/345 MPBBK rotors all round - here's how they look behind 461m alloys - whilst there's room I think moving the caliper out another 5mm/1/4" for the M2 rotors may be an issue.