Just flew Bangkok Airways PG0906 from Siem Reap to Bangkok, then connected on TG603 to Phuket on Dec 24th. The departure of PG0906 was 1:25pm, arrival to BKK at 2:20pm, then TG 603 departing at 4pm to arrive in HKT at 5:20pm. The layover time in BKK was approx 1 hour 40 mins. Which was more than enough time to pick up our TG tickets, clear Thai Immigration and hang out at the TG lounge!

FYI, Upon arrival in BKK you will see signs for Transfers, you will be lead to Transfer check in counters. Look for the TG counters (there are counters there for international transfer and domestic transfer). Once you get your TG bording pass you have to clear Thai immigration. FYI immigration was really quick, only 1 person in front of me and there were 6 counters open and staffed, plus this was the day before christmas, so I assume that there is a lot of travel traffic. SO long and short of the story is that 1:20 mins is more than enough time to get your tickets and to clear immigration in BKK.

Note* It is important to mention at check-in at Siem Reap that you have another connecting flight from BKK and to show them your e-ticket/itinerary with TG so that they can mark your baggage tag with your final destination airport code.

Note ** I was travelling with another TG ROP Gold on a paid C ticket and I asked the TG counter staff for a voucher to clear immigration more quickly and they told me that they don't have a express immigration lane for domestic transfers. I also noticed that they don't have a APEC ABTC lane for domestic transfers. All in all didn't need any additional privileges as immigration was really quick.
FYI, there is a TG lounge after you clear thai immigration for domestic flights.

Enjoy your trip to Siem Reap!