An R9 280X has been added to the hardware comparison page. With OpenGL, it's either as good as the original 7970s or better. At this point, it seems to be way better than the 7970s. It looks like whatever they changed has fixed the original 7970 issues.
https://litecoin.info/Mining_hardware_comparisonI get 750 kh/s with my Sapphire 7970s (w/ default firmware, 1080 engine, 1500 mem, I 13)
They're getting 740 kh/s on the 280x at 1100/1500. Obviously the same chip...so I wouldn't expect much different I suppose...but still a bit disappointing.