I am impressed by the hasrates you all seem to get. None of my R9280Xs go higher than 710 (and they get very hot, above 90 all of them and as a result the speed is sometimes reduced as low as 550).
I guess it is basically a heat problem... But it would be nice to be able to get 750 out of every card, it would mor eor less an extra card for me.
What's your config look like? After a bit of tweaking I have got mine up to 900+KH/s with a temp around 84 - 86 depending on fan speed.
900 with a 280x? Hard to believe...