I would not compare FF high ISO with the 1D MK IV. Compare it with the 1DS MK III or 5D MK II which are FF bodies, and already better than the 1D MK IV.
I wouldn't, either. But...Canon is doing exactly that, which is clearly a marketing ploy, IMO.
Clearly Marketing I agree. However I find all this approach a bit confusing because with their Marketing approach, it is a little vague how the camera will actually stake up against existing Canon FF camera (like the 5D mkII). I know we can estimate it, but until we see image, it will remain an estimate.
My personal benchmark for the 1DX ISO performance would be to compare it to the new Nikon D4, which is apparently 1 stop better then the D3s.
1DX, 24mm f1.4L II, 50mm f1.2L, 85mm f1.2L II, 135mm f2L, 24-70mm f2.8L II, 70-200mm f2.8L IS II : D800, D4, 24 1.4G, 35 1.4G, 50 1.4G, 85 1.4G, 24-70 2.8, 70-200 2.8 VR II