I have the same problem with a very similar card.
Card is Gigabyte 7970 Ghz edition.
In addition to what Benjie did, I also flashed the 7970 OC version, switched to Linux, changed memory, removed one GPU card, switched to all Catalysty 12.8, 13.1, 13.4, 13.5 (beta) , 13.6 (beta), and SDK 2.7 and SDK 2.8, cgminer 3.11.2 till 3.2.2.
Still I can't get more than 540 kh/s.
what kind of psu are you using and how many gpus per rig?
Lol. My 7970 runs average of 750 KH...
cgminer.exe --scrypt -o stratum+tcp://xxxxxxx -u xxxxx -p xxxxx -I 13 -g 2 -w 256 --thread-concurrency 8192 --gpu-powertune 20