You can also take a BUS from the airport to Orchard Rd.
If I had only 8 hours, I would choose to sleep in one of those 6 hour block at the Transit Hotel in the airport and NOT break Customs. There is even a restaurant there.
ANd I assume Duty free shoping. No need to go into town unless you were going to spend a day or two.
Can I be so bold as to ask, how do you deal with the fatigue and jetlag?