I already booked a roundtrip flight from Singapore(where I'll be staying) to Bangkok.  I normally renew that visa at Jomtien Immigration, but suddenly wonder if I can avoid the trip from Suvarnabhumi to Jomtien and back by going direct from the airport to the main Immigration Bureau in Bangkok and shortening the turnaround time into hours instead of a day or two at Jomtien. Maybe even book the return flight the next day.

You must use the immigration office that corresponds to your residence area in Thailand. If that residence is the Jomtien area, you must use the Jomtien office. Passports there as I suppose you know are often returned the next day or Monday after a Friday. 

Thanks! Not what I wanted to hear, but what I needed to hear.