I kind of doubt that "ALL" are unlockable. Likely the majority are but I'm sure AMD isn't just throwing out the chips that have a defect in a shader cluster considering they can be sold as HD6950s.
right now i'm interested in power consumption and how much this card can reach 6970 stock frequencies as i heard 6950 have some kind of difficulty to reach 880mhz without volt increase
The default voltage for the HD6950 is 1.1v and it is 1.175v on the HD6970. Flashing the HD6970 BIOS onto the card should raise the default voltage appropriately.