this is what i was told about this when my clutch went out a few years ago.
lightening your stock flywheel only works for a short period of time, because you are taking the steel away from it, you are in turn weakening the overall structure of the FW. when the FW is designed for a certain car, it is fully blueprinted to be precision balenced and structurally perfect, anything less would be imperfect so to speak and not work properly after a certain time (break and shatter).
so if you want to lighten it, go for it, just make sure you get a scatter shield. or just save the money and get one that is warrentied by the manufacturer... aftermarket...