I fly through BKK airport at least once a month. I'm a photographer and whilst my check in bag is always under 20kg, my carry or is probably close to 15kg and has yet to be weighed by Thai Airways, Bangkok Airways, China Eastern or Air Asia. The backup plan for when this inevitably does happen is to take all the heavy items out and store them on my person - camera and biggest lens around my neck, other large lenses in the cargo pockets of my trousers, and other bits and pieces in my jacket pockets. It may sound ridiculous, but that idea early of a photographer's vest is a really good one. Make sure you have a jacket with large pockets and you can get a lots of heavier items such as regs through. Technically, you can also wear your BCD onto the plane. Just tell them you're nervous about crashing in the ocean and want to be prepared.
Public taxis leave from just outside the airport, are reliable and run on the standard meter (max. 400 baht into town). Early in the morning it's about 1 hour to the grand palace/wat po which are well worth a few hours. Great street food nearby at the ferry pier. Getting back out takes a bit longer as the traffic will be heavier by that time. BKK airport is ok, but 8 hours is a stretch. You can get a Thai massage there and it could be worth paying the fee to use the 1st class lounge. Actually, with 8 hours you could taxi down to Pattaya and dive (if you didn't have to fly again)!