Not that special. Seen ppl doing 800 on a 280x, but those are hard to find...
I get 773kh/s on my 2 280x dual sapphire's @1110/1800 (didn't up the voltage though).

On and when will you all learn. Those last few kh/s extra all comes down to the card and not your cgminer settings tbh. It all depends on the ram speed, core and how good your vrm's are. Asus and sapphire are by far the best brands to mine (msi lightning edition too). Both manufacturers use ram that can be clocked very fast. My 3rd card from gigabyte can't even breach 6050Mhz and couldn't even mine on default settings (instant crash).