I can tell you that my Gigabyte 280x gets a steady 730Kh/s and I have it undervolted to 1.075 and it stays around 73C, but I also bought mine for $300
can you please post the cgminer settings?
--thread-concurrency 8921 --lookup-gap 2 --gpu-engine 1049 --gpu-memclock 1500 --gpu-powertune -20 -w 256 -I 13 -g 2 --gpu-vddc 1.075 --failover-only --gpu-fan 85
I am also using SGMiner, but I don't think it really makes a difference. SGMiner allows me to use -g 1,2