go for the raw mode comparison at ISO 12800 and bring up the 5DIII, D600, and D800. This is enough for you to have a good look at the noise.
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What you see is that the D800 has slightly worse noise but big buckets of extra resolution. If you scaled it down, you'd probably be in very good shape.
Well, that is precisely what the D800 review says. Scaled down to 5DIII resolution, the D800 performs like or slightly better than the 5DIII.
On the other hand, why look at ISO 12800 only? 5DIII reaches higher ISO values, which seems to me is the feature the 5DIII is betting on.
In other words, and as you were suggesting in your previous post, Canon and Nikon either went for higher ISO or higher resolution. Graphs also seems to confirm this.
Also, I assume that D800 and D600 do not allow as high ISO as the 5DIII because noise at those ISO value would be unacceptable. Is this a wrong assumption?