i've flight with both AirAsia and Bangkok Airways and even if i'm nearly 5'90" x 82kg i didnt find the seat VERY uncomfortable....anyway in my personal opinion even if was very uncomfortable this do not justify to spend 9000 baht more for just 1 hour and so of flight.
For sure if it was a longest flight this would have been a problem.
The Nana area is quiet in the middle between the 2 Bkk airports.... minutes more or less you will always need about 30 mins. But in my opinion better to move little bit earlier because of the traffic...