My flight leaves Bangkok at 2am the day after my visa expires. I'm thinking I'll clear customs before the expiry. Do you think I will have any problems? An extension seems excessive for 2 hours in the terminal. 

You won't have a problem.

 

Even if you pass through passport control after midnight you shouldn't be charged for overstaying as the don't charge for 1 day overstays at the airport exactly because of this type of situation.

Edited February 16, 20179 yr by elviajero

No prob. Relax.

I had a 1am flight once and was caught in a very long 2 hour immigration queue. Finally got stamped out at 15 minutes past midnight. A grinning officer stamped me as overstay but there was no fine.

Many Airlines open check-in counters three hours before departure (and if it is Thai or even Emirates, it will be open permanently anyway). To be on the safe side, Just go there early and have a beer or two in the departure area after being stamped out of the kingdom on time before midnight