Yeah I imagine if you are on charter and have to return a boat, flying out of STT same day is probably impossible. We are land based and always make the trip from VG-STT for our early afternoon flight usually with time for breakfast and shopping. We always just hop the first ferries available. We were through customs in STT by about 9:15AM last trip.
We overnighted in STT coming home one year thinking it would make it less stressful but found that after spending time in the BVI really the last place you want to be is STT!