Gigabyte R9 280x Rev2 is best. It produces 730k/s while its undervolted to 1.075V at 64C . Sapphires and other makes are power hungry. No other make can come close to Gigabyte version.
LOL, what settings are you using at 1.075V?
Iv'e only managed to get 680Kh/s max, but mine is on 1.131V. When i had it stock 1.2V it barely reached 600Kh/s
Thanks
--thread-concurrency 8921 --lookup-gap 2 --gpu-engine 1049,1049 --gpu-memclock 1500 --gpu-powertune -20,-20 --gpu-fan 75 -w 256 -I 13 -g 2