Not sure why you're saying the KH you're getting is at "stock" clocks.
He said stock volts

Ah OK. But he did say he's getting ~720 on stock clocks from a 280X which I find equally outlandish?
The Powercolor 280X has a stock core clock of 1030Mhz. Factory-overclocked, I guess. Which is why I could only overclock them from 715Kh/s to 740.