The bigger the better, but usually with BBK's they take a few good hard stops or partial stops to get them warmed up. Depending on how long your Auto-X course is and how long you wait between runs, it may not be the best idea. It depends on your pad and rotor combo too. You 'may' need to run a bigger wheel up front to compensate the bigger calipers.
Have you looked into a upgraded pad and rotor set up on the stock SVT Brakes? I like my Centric Rotors and Hawk HPS Pad set up.
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06 Spring Into Focus Best Sedan/Wagon Winner
03 SE SVT Wannabe (sold) to help pay for my 08 Cooper Red Mazda CX7 AWD GT