I have Gigabyte R9 280x Rev2

I undervolted them 1.075

mining 720-730 for both cards.

This is the settings I use

cgminer --scrypt -o stratum+tcp://yourpool:3333 -u yourwoker -p yourworkerpass --thread-concurrency 8921 --lookup-gap 2 --gpu-engine 1049,1049,1049,1049 --gpu-memclock 1500 --gpu-powertune -20,-20,-20,-20 --gpu-fan 75 -w 256 -I 13 -g 2