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.
At a steady 60c here, I would be worried about lifetime of the card just to get 20% more bitcoin. It will take about a year for you to pay off a new card with that 20% extra. Probably just me.