How much is safe is best left to the person doing the machining, as a example a std 2wd cossie is 8.9KG, I recently weighed mine before lightening and skimming/pegging.
Mine was taken down to 6.6KG, today we weighed Rossies one and it weighed 5.3KG, but was costly.
Mine cost me about £35.
Cossie flywheels are very heavy, for reasons of NVH no doubt, can make them a lot more responsive lower down with a lighter flywheel though.
tabetha