5D2 was about 2 stops better (in noise) than 5D classical (each pixel is better, and almost twice as many pixels), resulting from 3 years of research/improvement.
Now, it is almost 4 years since 5D2. So Canon should have enough time to make the same progress that it made back when developing 5D2. So that is "at least 2 stops". 1DX appears to be 3 stops better at the pixel level when compared to 5D2, but it has a slightly less pixels, so over all, it is less than 3 stops better (if each pixel is 3 stops better).
