Different flywheels give different idle and pick-up characteristics. I always go steel flywheel, and never had issues. I did go alloy once with steel insert and yes it was slightly noisier, but that was with a paddle clutch. Its something you need to consider. I have always noticed a difference in pick-up but it depends how light you go.
Its not a power increase, its a reduction of parasitic loss, the same as under-drive pulleys. Some work to good effect and some dont/or to a lesser degree. Underdrive on my V8 and you gain around 15hp, so its worthwhile. The other benefit is if you are running at high rpms for long periods, the under-drive will return your driven items to within spec speed.
Both changes must be a considered decision/choice.