Just came through BKK airport ( and sat here now on my return writing this) ... I actually used a fast track service, they meet you at airport door cost around £30, as I needed to ensure I was through quickly, I was flying F, but didn't want to risk it. I was through passport control within 1 min, they took me through the diplomatic entrance. But yes, if flying business you can use the fast track also, just ensure you get in the right lane and don't let them sign you a different way.... unless of course it's empty.