A lightened flywheel can make a noticeable improvement in acceleration in 1st gear with its effect diminishing as you shift to the higher gears.

A lightened flywheel does not increase the brake horsepower the engine produces.  It only decreases the energy stored in the rotating components of the engine and will make a little more power available to the rear wheels WHEN the engine is accelerating very rapidly.  When dyno testing, a lightened flywheel will sometimes show a very slight improvement in power before the power peak if the dyno runs are under 2 seconds in length.  The dyno will not usually show any power gain by the time the runs are 4 to 6 seconds in length. 


If you are racing on pavement or where you can get real good traction and your swing arm is long enough to keep the front end down, it could improve your acceleration in the lower gears. The overall acceleration and lap times of 250cc or larger dirt bikes and quads usually suffers when the crank or flywheel is lightened.

Lightened cranks and flywheels always shorten the life of the clutch basket, clutch hub and transmission gears.