|A lighter flywheel can aid in spooling up the rpms faster. Road racers use them all the time because they aid spinning up the rpms to match gears as they up and downshift around the track. It's all about the rotational mass as a measure of inertia. All it will do for you is let you spin up the motor slightly quicker when you rev it at the stoplight.
But switching to a lighter flywheel just because you heard it'll be a performance upgrade is only half of the equation. The other half is that it comes at the price of smoothness and drivability off the line.
The heavier mass helps keep the engine smooth when the clutch is engaged, and for a street car IMO that is much more important than a minor increase in the rate of angular acceleration offered by a few pounds removed off the flywheel.