Hey man, I just looked into this myself, as I was wanting to up my GTX 465 into possibly a AMD 6950 (which I will unlock).
That being said I found this TechPowerUp
article pretty helpful.
"I did some additional power consumption tests and it seems that AMD's PowerTune power limiting system does not get "upgraded" by the BIOS flash. We see that even when modded, the HD 6950 (at HD 6970 shaders and clocks) runs into the PowerTune limit earlier than the regular HD 6970 which results in reduced power draw but also reduced performance in Furmark. With a power consumption of 202 W, the modded HD 6950 is well below the 225 W specification limit of the HD 6950 power configuration.
Once we up the power limit in Catalyst Control Center by +20%, we see that the modded HD 6950 draws 252 W, which exceeds its specification (225 W) by 27 W. This move also improves rendering performance by a good deal, so I would recommend it to all users who perform this mod and have a half-decent power supply. Going 12% beyond the specified power limit will not have any ill effects on your hardware and is well covered by manufacturing tolerances and overspeccing, but it reduces the possibility of stuttering or other performance slowdowns caused by AMD PowerTune."
If i read this right, nothing should change? It is in essence the same chip and with the appropriate shader unlock and bios flash, they should tune the same? The binning of the chips will vary of course; supposedly that's why a 6950 is labeled as such?