I'm using 4x 7970 cards, all on x1 > x16 extenders.
Was getting max 690KH/s per card, but with old MB, CPU and only 2 of those card I was getting ~730Kh/s.
I've changed priority of cgminer.exe process in Task Manager from Normal to High, and voila ~730 again per card
So just make .bat with start "cgminer" \high in front and it will run it in high priority all the time.
That's weird, does cgminer.exe use any cpu processing power for the 7970's?