Someone else can confirm that such rims are fit to the cts-v brakes without spacers?VMR v710 wheels, 18x8.5 et45 cleared the front 6 piston calipers with no spacer. It's a tight fit, but it fit.
VMR v710 rears also in 18x9.5 et50 cleared the rear 4 piston calipers.
I also have a set of 17x9.5 et54 c6 replica wheels with 5mm spacers that clear the rear wheels. Just had to tie back the brake lines for clearance. There are plenty more options for drag radials with a 17" wheel and 26" tire than 18" wheels.