I forgot the exact formula but for every pound of weight taken off a corner its roughly equivalent to if you were to remove 65-85 pounds of total weight from the car.
ie you start with a 3500 car with 25 pound 17's then you drop 5 pounds each wheel to 20. That is roughly like reducing the weight of your car 65*5 = by 325 pounds! I can look up the formula and its probably all over the web but its the centrifugal forces that do some damage. Lighter wheels also are easier to slow down as well. So in a nutshell light wheels increase performance, make the car feel lighter, and its a braking mod as well. That is what all the hype is about.
