Greetings from the TG Royal Silk Lounge in Concourse D.
Suvarnabhumi Premium applies for departing pax as well, provided you are issued a pass to use it. Mr. Megatop and I had one, and we used a dedicated Suvarnabhumi Premium departing Immigration channel separate and apart from the one I understand TG operates. The Suvarnabhumi Premium area was marked with the same logos as the arrivals area, and it is also located in the middle (as between east and west) of the departures hall.
As a nice little bonus, the end of the pass with the "Thailand Elite" logo upon arrival is currently replaced by a King Power 1000-baht coupon for departures, redeemable if you spend more than 5000 baht at the duty free shops. Mr. Megatop and I did, so we got a nice little 2000 baht from the program.
Swiss participates in it. We're departing on LX 182 soon.