I quite agree that good brakes are important and it is sure nice to think that one has the best available fitted to your motor.
Checking out the BMW UK 'build your own', the M Sport Braking System appears to have larger diameter discs than the std 'as fitted' to the 335i.
But I quess this cannot be so, if you say that the M Sport brakes are only 370x30 & 345x24. I am sure that the 335i are larger than this.
I am sure that there is someone out there who knows!