Counting everything: 6950 CF has 27.5% over 5970 and 42.8% over 580[/B]
That's 27.5% more power costing 3.4% more or 42.8% more power costing 20% more. To put it into perspective, 6970 costs 23.3% more then 6950 and only brings ~12% better performance then 6950.
Unless i made some mistake, i didn't arrive @ the same conclusion as W1zzard:
You calculated that the 6950 crossfire will do 142.8% of a GTX580 and 127.5% of a HD5970, but this is considering both cards full performance, the graphs show the HD6950 performing 100%, and following your math, the GTX580 does 70% of the crossfire, and the 5970 does 78,4%
A little different, but not that much.