I think what we are going to see is that the 5D3 is less forgiving of exposure than the previous versions. I think the trend continues where Canon has tweaked everything they can to get the performance they want, however, it comes at a price. That kind of sensitivity and processing will mean you won't be able to have the same kind of exposure latitude.
If it is the case, it doesn't bother me any since I ETTR as much as possible, so it would be no different if I shot with the 5D3. I hope I am wrong though. I looked through those FM images, and the ISO 1600 shot should be cleaner, but since it was a bit under exposed, the noise popped out. It feels a bit like what I saw with the 7Ds when I had them (and also with the 1D4 now to some degree).