I have 205/55/16s on my street wheels and 225/45/15s on my race wheels and the difference feels huge, ~10lbs a wheel/tire less rotating weight on the 15" too. I can't say that it actually is that much faster because there is a quite a bit more shifting required but acceleration everywhere feels a lot stronger. The downside however is 23" tires look funny on the car, cause you to loose a lot of ground clearance, and are too short for comfortable 80mph cruising.
