Maybe someone will find this useful:
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.