handling car 17's or 15's?

This may be heresy, but here goes. You may want to consider different profile tires, front and rear. If you look at major road racing series where tire sizes aren't rule bound, you may see that low profile tires are used on the front and higher profile tires are run on the rear. Reason: The low profile tires provide sharp steering response (less slip angle), the higher profile rear tires have a larger (longer) contact patch that lets them put the power to the ground.