I had an 1GB 6950 (single fan) which gave me around 440khs stock. Cant remember the voltage. Flashing to 6970 bios (i think this is what u meant) is not a good idea. U already got 1536 shader count (6950: 1408 /6970: 1536) so this will give you no advantage.
Maybe you can edit your stock bios with RBE and flash it back with changed voltage.
Did you try to change the voltage with MSI AB or Trixx?