Honestly, there is no way a heavier flywheel is going to add anything, torque or horsepower UNDER THROTTLE, ain't happening. It's just the opposite, it takes more power to accelerate and to maintain the rotating mass. Off throttle, yes, the flywheel will then transfer it's stored energy back to the drive train. Anything our motors have to rotate, chains, sprockets, wheels, tires etc... The heavier they are the more power it takes to turn them and the flywheel is no exception.
The positive side to a normal or heavier flywheel is the ability of the engine to maintain RPM, especially when changing gears. The lighter the flywheel the quicker the drop in RPM when off throttle.