Thai are giving an extra 10kg per ticket on all classes currently, which helps. They also tend to not mind if you're 5-8 kg over in total from my experience. Link is here, perhaps print it and bring to checkin counter.

http://www.thaiairways.com/en_TH/new...xtra_bag.page?

Also who is your HKG-SFO flight with ? United? Sometimes you can through check across 2 Itin and benefit from the one with the largest weight allowance.