i got the dual bios with dual bios switch on my sapphire 6950 3L and it did not unlock,and i tried every possible solution (took me 5 hours to do all of them).The manufacturer cut it out,it has dual bios,but its "double BS".
It is like playing roulette...If i were you i would go for 6970 which you can get used in very good condition for about 280...