IMHO better to stick with 580 MH/s underclocked using 20% less power and keeping the heat down)
Why's that? Using those numbers the performance scales linearly with the power draw. Airflow in my case is a mess because the manufacturer included less then half the fans and they're not configured for multi card setups. With the side of the case off I'm getting a steady 78c OCed. 20% more coins now will add up as the difficulty increases.