I went on Tuesday afternoon to extend non O-A, the place was mobbed. Perhaps because of this, when I arrived around 1.30pm and went straight to the ticket desk I was only asked for the TM7 and then given a queue number right away (119 ahead of me in the queue, last year there were less than 20 queueing at the same time of day). I popped downstairs to Krungsri which was very quiet, made a deposit and got the book updated was told to come back in 40 minutes for the letter, which gave me time to fill out the additional forms. Then got copies made, and hung around getting coffees etc. Finally got seen after 6pm, unusually by a senior officer who checked my forms and stamped my passport all at her desk.
Very smooth trip, but surprised at the queue length in front of me, took over 5 hours in all. The QR code provided on the ticket was great though, showing queue number status, and freeing me up to wander all over the building rather than sit glued to the screen in L section.