OK - there is something different about my card. Here are the defaults:

core voltage: 1025 mV
core clock: 950 MHz
memory clock: 1425 MHz

540 MH/s

Two things are odd. First the default core voltage seems lower than 2x0ninja and others I've seen have. I can set it to what his default is (1170) and that allows me to get to a GPU clockspeed of 1100. At that I'm getting 590 MH/s - but as I increase the voltage or clock from there the has rate moves up and down over a range of 50 - 100 MH/s but ultimately leaving me at a lower rate.

core voltage: 1170 mV
core clock: 1100 MHz
memory clock: 1425 MHz

590 MH/s

I also don't understand why my memory clock is 1425 when in forums everywhere they talk about it being the core clock - 150. Mine is higher - to no benefit. Maybe they've recently changed the design? It may present a new tuning problem. Varying the memory clock doesn't seem to make any difference. FloridaBear - I do get around 520 for the same settings you are seeing 584, yes. Weird. :-/