I have LOT'S of RAW files from the 1DX, 5D3, D4, D800, D800E, I took them all myself during a photography stand here in Norway.
I have been trying to compare (in a non scientific way) shots done with all cameras of a similar scene, with different ISO's, and also underexposed down to EV -2 and lifting shadows after.
The 1DX is maybe 1/2 stop better than the 5D3. About same quality as D4.
The 1DX might be slightly better in the shadows than the 5D3, slight less banding.
The D800/D800E is impressive taking into account the huge resolution,
still nowhere close as being that much better than the 5D3 in the shadows as some tests say.
The 1DX (and the D4) is a pricy camera, but I think you pay for getting speed and AF, not just SNR/DR.
IMO all cameras are good on ISO/DR, and the differences are so small it makes little sense for anybody
to claim any camera is substantially better than another. IMO, at ISO25600 things are starting to get useless,
regardless of what camera you use (maybe at ISO12800 on D800).