For the voltage issue, download MSI Afterburner, and under settings click the box that says "Unlock Voltage Control". Not all cards can be vddc adjusted by default.
Also, 595 isn't bad. I have 4x Sapphire 7950 4L's and seem to cap out at 595 myself. I also have 16 PowerColor 7950's that push 620 KH/S with the exact same settings. The Sapphires (mine are reference blow fan designs, which if I am not mistaken yours are as well) run significantly cooler that the PowerColors at the same clocks/cddv, etc. 10C cooler at 90% Fan actually. So 595 really isn't that bad when all is said and done.