15's will not fit over the stock rear calipers though they can be modified by grinding off something...don't remember what. You have to change to rotor as well as some other stuff. Chase Kirby at Mirror Image (see Facebook) has a kit.
2009 Hot White G8 GT, CTS-V v1 fronts, Camaro 20's, BMR Sways, Eibach Springs, Rear Cradle bushings. GXP FE3's, BMR rear susp. Kooks 1 7/8 LT headers, RCR custom 3” exhaust, ported LSA blower, 2.38 pulley; Ported heads and TB; LS7 MAF, Racetec pistons, Scat rods, CTS-V crank; FAST 85 injectors; RCR Lunati cam: 219/237 .612/.601, 114 +2 LSA. CTS-V pump/VaporWorx cntlr. Built trans, Methanol injection. Rick Crawford built and tuned. Best 1/4 9.68 @142, 1.42 60'.