Will make the idle a bit lumpier, and as said does not store as much energy, so the revs drop quicker between gearchanges due to less inertia from flywheel, and so you get a better gearchange, quicker pick up also as it is not just the weight of the flywheel that you are having to combat, you spend energy accelerating it, and at 6000rpm it weighs a lot, just the same as balanced wheels make the car go faster than unbalanced ones as they need less energy to rotate a balanced object.
It will not be as smooth to drive especially low down due to cyclic vibration caused by engine firing pulses, which is the primary job of weight of a flywheel to smooth out.
A good mod for track use.
tabetha