You probably would see a small performance gain. I know with tires it's something like however much weight you're turning is like having double that sitting in the car, eg: 4 15lb rims is like having 120 lbs sitting in the car. I don't know what it works out to being on the crankshaft but the principle is generally the same. It's just less weight that you're trying to move and so more power available for moving the car.
2006 Red Chevy Cobalt LT
mods: Kenwood DNX5140 Head Unit, HIDs, Halo headlights, LED taillights, Injen CAI, LSJ throttle body, 2.4L intake manifold, red interior LED's, TWM short shifter, custom fit red seat covers, red trim and door handles, SS gauge cluster, Racing Solution long tube header/downpipe, Corsa touring exhaust, lightweight crank and alternator pulleys, trifecta tune, 17" Motegi black rims