Couple of comments regarding BBK's in general....
- I had stoptechs on two E46M3's (fronts only) and they were great on the track and pad changes were easy. Additionally, the pads were the same as an early 90's Porsche so there were a lot of pads from different manufacturers to be had. I had a very good experience with this setup.
- I don't understand two things - first why both stoptech and brembo are pushing 4 wheel setups. I understand cosmetically it's appealing but performance wise - it's a waste of money. As a matter of fact, before stoptech started offering 4 wheel setups they had a white paper on their website saying 4 wheel kits were overkill.
Secondly, why does our car need a 380MM rotor? Previously, the E39M5 BBK setup was a 4 piston 355MM setup. This car weighs approx. 300lbs more than the M3 and I've seen people use a stoptech front only 355MM setup with R compounds on the track with no issues.
I suspect that the OEM problem is more the thermal capacity of the pad not the rotor. The pads on the front of our car is relatively tiny when compared to a BBK setup. I think the thermal capacity of the OEM rotor is probably ok - it's the pad that's the issue. I think this is supported by some people saying they just upgraded the pads and fluids for track use with no problems. Obviously this is dependent on the track, amount of grip from the tires and braking technique.
So it seems that a front only 355MM BBK setup would be more than adequate for track performance especially in an HPDE environment. Personally I'd be happy if someone came out with a caliper setup that used the OEM rotors as I agree $7-$8k for a BBK is ridiculous.
Just my $0.02....
'15 M3, '16 911 Turbo S Cab