I suspect the weak spot in the brakes would be not the calipers but the pads compound and cooling. I would go with Hawk DTC70, and would try to fit the 3" brake ducts, maybe two on each side if you can make them fit, maybe with active blowers (I've previously used 4" blowers, then narrowing into 3" ducts but I'm not so sure that they were very effective, they seemed slightly useful but not hugely so).
The stock brake pads on GXP are actually not that bad, and I suspect would last a couple of 15-minute sessions (not a race though). I haven't tried driving G8 on the track, my actual track experience is with the older Camaro and Mustang, which are lighter cars, with less power. The Hawk HP+ autocross pads mostly lasted a lapping day on Camaro, tending to start melting in the 4th session of the warmer days.
I've tried Cabotech (I think it was their 11th-grade compound or some such) and DTC70 on Mustang, with brake ducts. And once just the street pads from NAPA, when I had no time to get anything better. The NAPA pads lasted about 10 minutes of practice. Both Carbotech and DTC70 lasted multiple sprint race days, with the difference being that DTC70 works right away (and I wouldn't hesitate to use it on the street), while Carbotech requires a lot of heat to work, so I was spending the warm-up lap and first half of the first race lap warming up the brakes. Carbotech works great on the light cars (I used it on Mazda-323 too) with softer compounds but takes a long time to warm up on the heavier cars with harder compounds, and I definitely wouldn't want to use it on the street.