I was thinking the same thing.
Why is there only 1 piston on M cars???
I think it's interesting because there is so much conflicting information here. I'm going to give my opinions but it is only based on my recent experience.
I had an e46 M3 and the brakes were fine. I never drove it on an auto track so I really didn't even think twice about the brakes.
Now I drive a 335 and the brakes are bigger. The calipers are bigger and the rotors are bigger.
Well, I recently purchased the rotora caliper kit. It has 6 pistons in the front and 4 in the rear. I can tell you right now that the braking in my 335 is unbelievably better. The pedal feel is better, the stopping power is better, and there is no fade. I know that there is usually no fade when driving on the street, but what I mean to say is that the brake pedal feels the same exact way every time I push on the pedal.
The person that said the comment about the tires though is correct. People were telling me that once I get this setup I would have to get stickier tires to keep up with the braking power and they were right. If I stop really fast, I'll hear my tires screeching, but they aren't screeching because they're locked. They're screeching before the ABS kicks in.