It's because
Hi,
I have one of Sapphire R9 280X OC Dual-X on Ubuntu 13.10 but the kh/s don't want to break 715. I tried all configuration on this thread without results.
Any ideas?
Hashrates rarely go above 720 with a single card. That's actually pretty good, most single cards run 680s-710s, sometimes lower. You need more cards to get full speed on all of them.
Referring to Windows, on Linux it's easier, since you are running that try playing with the settings through SSH if you can since desktop usage can cause lag.