do not talk rubbish here.
I have done that once. You can be at airport aroud 6:40am start from nus by MRT.(the first MRT is around 5:40)
9:30's plane, be there around 8am is early enough, take 6:20's bus 200 from Pgp to buona vista. use about 20 minutes. take MRT from buona vista to changi airport need at most 50 minutes, so 7:45am you are at airport.