GT-RS is too small and will produce a lot of heat. Something bigger would be better for road racing. There will be sacrafices to be made either way though. You want fast spool? Get a small turbo, good for the street...but don't get upset when your EGT's are through the roof after 20 minutes on a road course. Get something a little bigger (not big, just medium sized), less streetable; but you'll produce less heat, more power, and at less psi! You won't have to worry about spool up since you're always at high rpm.
If you have lots of cash, go with an HKS 3037 kit. You could get the Greddy T67 kit too. Look at Apexi. Or build your own kit with something like a T3/T4, new Garret GT turbo, or even a mitsu turbo (close in size to the T67). Just my $.02.
