Getting a flywheel lightened and balanced (through machining) can give you a more free reving engine, ie improving the engines response to throttle openings, however this will NOT increase power output. In addition, tuning an engine usually results in a shift of power up the rev range, losing torque at low revs, a lightened flywheel will greatly exagerate this loss of torque at low revs decreasing driveability.
Lightened and balanced fly will actually loose you torque rather than gain it.
Exactly what popped into my head!
