I think a lightened flywheel is an area a dedicated modder can take good advantage of. The main reason for stock oversized flywheels is driveability and idle smoothness. But many hardcore modders don't hardly idle (because it is a complete waste of energy) and can learn new driving techniques, i.e. launching efficiently with a lighter flywheel. And may already time obstructions and lights to conserve vehicular momentum. As soon as you get off of idle and have the clutch engaged, the big flywheel doesn't offer any advantages and is holding your pistons and your car back.
If you P&G a lot then you are straining your engine against accelerating a heavy flywheel only to throw that energy away at the end of each shift and at the end of each pulse. It should be less strain to bump start a lighter flywheel as well.
I'm going to try a lighter flywheel in the tdi golf once I get around to fixing it rather than synch shifting it everywhere
