So for my cards the 3 XFX HD 7970 Black edition coreclock 1000/1425, i founs a Bios sollution , And I have flashed them all.
When flashing as somebody asked you must flash with the Bios switch in position Nr 1
Pos. Nr 2 is locked, and good to have if ya screw up...
Core is then reset to 925/1375, then you have to manually overclock to 1050/1575
and i now get a hasrate of round 740 kh/s

The Bios that I used Is one which is Modified by "TheStilt" and it will 100% work on the cards that have the part number "FX-797A-TN FX-797A-TNBC v1.0"
on the back sticker..
heres a link to that Bios http://s000.tinyupload.com/index.php?file_id=73769960143386079737

It may also work for other similar cards but I dont know, but as long as you have a bios switch there shouldnt be any problem in trying, at your own risk.

https://dl.dropboxusercontent.com/u/27596325/test3.png