A forum thread at DPReview was recently removed that showed a less-than-scientific dynamic range test of the EOS-1D X Mark II. An image was taken and underexposed by about 5 stops and then corrected in post. To most eyes, it showed a 2-3 stop improvement in dynamic range when directly compared to the EOS-1D X.
The Canon Rumors forum has a lengthy discussion about the findings with a full resolution comparison, and we don't have to delete it for NDA reasons.