go for the 1 piece flywheel to lighten, ive had one of those 3 piece ones and theres hardly any weight to be saved safely.
fyi if you check my blog i can get the stock cg13 flywheel down to 4kg (from the 8.4 ish standard) without drilling. but my aluminium flywheel is 3kg without drilling, motorbike cg13 anyone? lol