You will need to do a rather unsafe overclock on memory. I have core voltage at 1080 (default 1200) with the core clock set at 856 mhz (performance drop at 857 or higher). Then tuned the memory voltage between 1625 to 1630 (default 1500). When the memory is overvolted you can turn up the memory clock to 1775-1785 mhz. Be sure to run Intensity at 13 in cgminer. If you set any other intensity with these settings it drops 100 kh/s per gpu. I believe these ghz editon gpu's are going back to the store in my case as they are horrible for scrypt mining.
The speeds seems to be exactly same with 1333mhz, and 1600mhz RAM. Please PM me if you can find any settings that perform better. Hope this helps.

The hash difference is likely to be the Core clock and Memory clock speeds. Post what yours are set up at because those flags that have been posted are under the assumption you've overclocked which is rather normal to get above 600khash/s.

I would really like to know this as well. I have the cards performing at 595 Kh/s after a few hours of fine tuning but they are really sensitive to any changes. Changing to any aggression except 13 throws the cards back to around 400 Kh/s, or lower.