I successfully received my visa extension at Chaeng Wattana yesterday, based on B800k method.
But a warning about obtaining the account balance letter from Bangkok Bank. I arrived at Immigratin at aound 13:15 for a 14:00 appointment, allowing 45 minutes to get the letter. It actually took over an hour because only 1 window was processing them and the teller was doing other work as well.
Luckily immigration allowed me to use the appointment desk and I had no further problems