pbonsalb wrote For about $2175, you can get a Stoptech ST40 kit with braided lines, pads, and hatted rotors that is lighter than stock and should stop just as well or better. Cost is less than the refurbished Brembo kit with new stock rotors.
http://www.bimmerworld.com/Brakes/BMW-StopTech-Big-Brake-Kits-BBK/StopTech-Big-Brake-Kit-Front-M308-355X35-ST40.html
6 piston kit with 380mm rotors is $2600:
http://www.bimmerworld.com/Brakes/BMW-StopTech-Big-Brake-Kits-BBK/StopTech-Big-Brake-Kit-Front-M309-380X35-ST60.html
New, not refurbished. Lightweight 2 piece rotors with replaceable rings. And designed to fit the rotor perfectly. Cost is a little more than the refurbished Brembo rotor kit with new stock size stock calipers.
If you just bought new rotors and are now looking at a BBK, the refurbished Brembo kit is worth a look. The other company selling new Corvette Brembos that are machined to fit adapters and use stock rotors is also worth a look.
it's little bit strange to compare different (very different!) size brakes.
you need to compare 4 pot stoptech kit with oem brake system, they are on the same level.
and, to compare 6 pot stoptech caliper, you can check pads dimensions - it has 152mm brake pads. it's closer to 4 pot brembo (132mm length), than 6 pot brembo, with huge 190mm pads.
8 pot brembo has 2x 95mm pads, which is almost the same as 190mm 6 pot pads.