I dont think its possible to undervolt the Gigabyte cards? They are overvolted stock at 1.25v I believe.
Would down-clocking the memory speed lower the hashrates on the cards?
For bitcoin mining you don't need the GPU memory speed. Higher GPU Core Clock drives my hash rate higher.
If you are using scrypt for something like litecoin that is another story.
... oops! you are going for LTC my bad!