Well, after lurking these forums for a while, I made an account to share my similar plight with the Sapphire 280x Vapor-X throttle issues. I have tried a variety of configs and the best I can achieve is ~710kh/s with 64C. Even this can be a little wacky and drop down to 695kh/s at times. Regardless, I have noticed the same throttle issue on my card after 72C (yes, it is bios version 015.041).
Anyways, as I'm sure most you have figured out by now, we need to find a way to bios flash to 015.039 if there's any chance to start mimicking the results many of the other Sapphire 280x Vapor-X owners have successfully attained. Unfortunately, I have tried various Vapor-X bios versions from http://www.techpowerup.com/vgabios/
to no avail. In a last ditch effort of desperation, I have tried Sapphire Dual-X and even other manufacturer bios' but then I get the above-mentioned issue whereby the CPU won't boot and POST. Essentially, I had to save the backup bios with GPU-Z and then reload it with ATIWINFLASH.
I hope someone can nerd-rage this problem as I'm sure many Sapphire owners are having this problem at the moment. In case anyone cares, this is the config I'm running at the moment.
setx GPU_MAX_ALLOC_PERCENT 100
cgminer.exe --scrypt -o stratum+tcp://ltc.give-me-coins.com:3333 -u FUCK -p SAPPHIRE --thread-concurrency 8192 --lookup-gap 2 --gpu-engine 1027 --gpu-memclock 1500 --gpu-powertune -20 -w 256 -I 13 -g 2
FYI: My rate climbs to ~690kh/s right off the top, but then slowly rises to ~710kh/s over the next 5 minutes...