I've read that too but doesn't the 6870 reference get more shaders through a bios mod or something that get it similar if not more than the 5850?
That's the 6950 with it's 1408 shaders, flashable to be a 6970 with 1536 shaders.
Except it's a huge gamble whether or not the shaders will be unlockable or function, even on a reference board. I have 4 XFX 1GB 6950's and not a single one opened up locked shaders for me.
Don't go for the 6950 counting on making it a 6970, the odds are well against you.