Ive run a few beater cars that I was just screwing around with in the past and gutted them. Now when I say gutted, i mean gutted. Just bare metal, also the engine had about 80 pounds taken off.
There was about half a second diffrence between the before and after, this is in an old half destroyed fox body.
Not big enough of a diffrence to have the horriblely noisy ride. (a rock hitting the underside on the freeway sounds like you hit a mack truck.)
Honestly, the only time weight makes a big diffrence is when you get to the thosands of pounds.