No - you can connect between any two intl. flights in BKK without clearing immigration - it may be a 15-20 minute walk between terminals though - but it is all airside.

Dedicated security for each "pier" of gates makes security lines more managable also.

I would suggest BKK for the transit - rather than SIN - for this reason. I've done DEL//BKK-NRT-SFO before with similar timing - it was fine.