I have done this transfer from BKK 6 times. 5 went off fine. Many times the 12:25 arrival ends up being about 12:10 or so, do to padded schedules. Last month the flight was going out about 25 minutes late, so Cathay walked me over to Thai and put me on their 8:am flight. Next month I'm on the 6:40 because of pricing. You should be ok.