Set thread concurrency to 9984, gpu: 1100MHz mem: 1200MHz I:17-18 if it's a dedicated miner.
Remember to disable windows aero etc. If you're using it at the same time, even to open the start menu then lower the intensity.
I get little over 360KH/s with 1% of garbage in HW: & R: per 7850.
A few more KH/s in debian squeeze 6.0.7-amd64 with self built cgminer 2.11.4.
On another partition i tried it with win7u 64x with the prebuilt cgminer 2.11.4.
On both OS I used Catalyst 13.1, AMD APP SDK 2.7 and ADL SDK 5.0 ...obviously the adl in windows was whatever version it was because it came in the prebuilt cgminer.
I't also runs for a while at +390KH/s at gpu: 1200MHz and mem: 1300Mhz
...but wouldn't recommend without liquid cooling. And what ever you trial i'd say leave the voltage lock untouched.