reduces rotational mass... which
translates in less energy stored in the flywheel. when you accelerate the mass, you need to perform work, which is basically the energy required to bring the flywheel to a certain rotational speed. this energy is a waste, because it can't be used in any way... it gives it a high rotational momentum, which is basically, why the engine seems so lazy.
Another diasadvantage is that the energy you stored in the flywheel, let's say you're engaging the clutch ... and hm... for some unknown reason you didn't rev match... that energy will transform into ... either heat or a jolt forward... depending how smooth the clutch engagement is... the first is bad. uh... so is the second one.
so get rid of the heavy flywheel.