Having owned a 996 GT3, if that is your threshhold for satisfaction, sell the M3 now, it ain't ever going to feel close to the GT3. I have the StopTech ST-60/ST-40 system, and it's very good. But in the GT3, the hydraulic system, steering, and light front end all contribute so much to the perceived feel of braking, the M3 simply cannot be modified to compare. I will say the ST system delivers a firm pedal that stays consistent throughout a 20 minute track session.
2011 Space Gray Coupe, DCT, ZCP, StopTech ST-60/ST-40 BBK.