[Harpman]

To accelerate your car from 1000 RPM to 6000 RPM requires your flywheel to do the same thing. A lighter flywheel is easier to accelerate, allowing more of the engine's torque to go to your wheels.

As you approach top RPM in any gear, you are no longer accelerating the flywheel so the benefit then dissappears, so for instance your top speed will not increase any more than if you took the extra few pounds out of your car by removing your carpets.

Some racing Stock Cars have a very light flywheel to help with max acceleration out of each turn, but they practically stall when standing still unless the driver keeps the rpms up, because there is not enough stored rotational enery in between piston strokes.

So you may need to set your idle a few hundred RPM higher.

Bottom line: you get some acceleration improvement (which is great of course), but no noticeable higher top speed.

Also as Drew1 said: "the slightly bigger rpm drop when changing through the gears" this could be annoying. I haven't done this flywheel change myself, so get some more opinions here from folks who did before you go for it.