What cgminer switches are you using?  I've found setting vectors to 1 gets me better rates on my 7970 as well.

-v 1 -g 2 -w 256 -I 6 will get me about 660MH/s when overclocked to 1100Mhz core and 950Mhz clock.  The clock settings are pretty irrelevant, but lower it can help with power consumption.