"xintensity" : "4",
"gpu-powertune" : "20",
"gpu-threads" : "2",
"shaders" : "1792",
"gpu-fan" : "0-100",
"auto-fan" : true,
"temp-target" : "70",
Those last 3 lines are important, as the sapphire cards love to auto-throttle if they get too warm. And my 280x's are even worse.
1) no --thread-concurrency or --worksize switches in your config?
2) the 7950 has a core stock frequency of 925Mhz (at least my Sapphire card does), so you're quite a bit overclocked there at 1055. Not sure why you're saying the KH you're getting is at "stock" clocks. Or were you referring to your 280X?
Yeah, I only meant stock clocks on the 280Xs. And the point of xintensity is that you don't use thread concurrency. It's used as a multiple of shaders.