Without any overclock, a 7970 should be getting more than 520 khash/s easily. I have a 7950 boost (925/1250) and I'm getting more than 520 khash/s.
I know. It's a known problem with some cards, especially GHZ edition. mine is 1050/1500 stock, and is only 500-520kh/s.
Lots of people solved it by flashing non-GHZ BIOS, but I can't find one that will work with my card
Hopefully someone with the same card will share a good BIOS for mining.