Thanks for the helpful info everyone, esp. regarding expedited immigration at BKK. We should be fine--our Air Asia flight arrives BKK at 3:20pm so with a 7:30 departure on TG we have lots of cushion, but hate to wait in lines!! Would have loved to have flown TG from DPS but that would have meant a 23hour layover at BKK--not worth it for us--we'll be back to Bangkok soon enough.
That 23 hour layover in BKK may be what you need...
Cos with LCCs, if a flight's delayed, its not by a few minutes.. it could be a couple of hours! Not to say that Air Asia will be delayed.. but it has been known with Murphy's Law...