Many people say that the standard brakes are fine for the road. But I have never found that. Never. They don't warp, as is often misunderstood but merely overheat the pads causing uneven deposits to build up on the disc face. It is possible to use the standard sliding pot calipers on the track without any serious problems, provided the pads are changed to something like a Pagid RS19 or 29, but they're still not comparable to a good set of AP six pots.
Case in point; my CSL has the 6-pot AP's whilst my Z4M Coupe is fitted as standard with the CSLs standard brakes. Driving both back to back there is a massive difference in braking competence.
Perhaps if you measured absolute stopping differences there would not be a big difference, but the improvement in feel, bite and the ability to modulate braking progression is so significant that nobody would have any question over which was preferrable. Driving with standard brakes I find myself saying "they are ok..aaay", but with the AP six pots I immediately feel they are everything I could want in brakes and more. They match the competency of the rest of the car, whereas the standard brakes just aren't as impressive as the other dynamic capabilities of the car. They are a 'compromise' and that sticks in my throat.
Be in no doubt that when we test the new M3 in EVO we will be pointing a big finger at the brakes if they are anything less than impressive.
Interestingly though the new 135ti that is doing the tour of BMW facilities at the moment is wearing a big set of Brembo 4-pots with 355mm discs. It will be launched in less than a year and will make the M3 look a little silly if it is fitted with the better stoppers...