Typically straight line traction will be worst than with 17s. Even though your tire will be wider with a 9.5 wide rim the tire will have a shorter sidewall and not be able to absorb the shock of dropping the clutch. Improved grip even on a road course is debatable with larger daimeter rims, generally what is preferred is the smallest diameter rim to clear the brakes. This reduces rotational mass and allows the largest sidewall to absorb bumps better.
On another note, 18s look very pimp. I loved the look on my Saleen, but gave up some performance for them. At the strip my best times were with a set of 15inch stockers with drag radials on there.
there is some one on here who has the set up you are looking for. if you did a search you can find it. He just added the 18's to the rear. there is also a guy who has the 18's all the way around. Sorry i cant remember names but i know it was in the mods sectiono.
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