Hash speed seems to scale linearly per shader per MHz with proper memory speeds/settings/sdk/etc. With 735 core and 1440 VLIW5 shaders, you should get about 307 MHash, so you're pretty much close to the mark. I can suggest to run 205 MHz memory and 128 worksize (About 0.15% mhash loss, but the lower temperatures makes up for increased core speed headroom. I got about 4 watts less per 5830 going from 330 memory to 205 memory, about 1c less in temperature and allowing a small bump in core) and cranking up the core speed.
If you're wondering about where I got the memory speed from, I was bored one day and did an exhausting test of many memory mhz vz worksize. you can see it here
https://docs.google.com/spreadsheet/ccc?key=0AjXdY6gpvmJ4dEo4OXhwdTlyeS1Vc1hDWV94akJHZFE&hl=en_US#gid=0 or in this picture of a graph.