I don't think this is optimized, but I'm getting about an average of 715Mh/s on for my 2 7970s @ 1180/110 with this config:
"vectors" : "1,1",
"worksize" : "64,64",
"kernel" : "poclbm,poclbm",
IIRC, those are the CGMiner defaults for a 7970 anyways, so I doubt you would get any better than that.