This card is already close to the maximum number of watts than can be handled by its VRMs and the PCI-e connectors/cables. I would not recommend increasing voltages to try and get more kh/s something will fail somewhere. Remember that in the low-voltage high-current world of GPU's even a tiny increase in voltage can result in a much increased power consumption and hence thermal output (Power = Volts*Amps). You'd be much safer with 2x 7970's.