I went w/ stainless steel lines for more pedal feel, DBA slotted rotors, and hawk ceramic pads plus flushed the fluid. This is a major difference over stock, and unless you're autocrossing the car more than you will need. The G8 GT even in stock form stop ridiculously well...like german super sedan well. 60-0 in as little as 109' has been documented, compare that to ANYTHING and you will be impressed. It is the fade and feel that is a problem w/ the stock setup, w/ the mods I listed that will help in those departments.
A larger caliper w/ more pistons will always be "better" but not necessarily needed nor will it yield better "panic stop" results in the real world.
'08 GT sport/premium, tints 20/35, 18" TSW nogaros w/ bridgestone blizzak winter, 19" sport oem w/ hankook evo summer, Philips crystalvision bulbs, debadged, Vararam OTR, JBA 1-3/4" LT's, ported TB, MF axlebacks, MF X-pipe, DBA-powerslot brakes, russel SS lines, hawk ceramic pads, GXP shocks/struts, lovells poly strut mounts, Slowhawk Tuned 344/356
Gone -'04 GTO A4 2.5" custom exhaust, sweet thunder muffs, H pipe, Z06 heads, Baer brake it, 20% tints, torrid red, AEM brute force intake, tuned