In this case, if 1DX@51200 = 7D@3200 then DxO tells me the 1DX should be 1 to 2 stops better at high ISO (using the SNR graphs) than the D4/D800E. Now if that holds true and is present right up/down the curve, then the 1DX represents not only a new sensor but newer sensor technology than is in the 5D3.
I would like to see an ISO 51200 real world comparison to the 5D3: same location, same lens, same exposure value...
Just take a look at the side by side comparison in the 1DX review posted on the CanonRumors home page today. It provides exactly what you are asking for. The author provides high ISO sample photos of the same subject shot with both the 5D Mark III and the 1DX, and then puts 100% crops right next to each other so you can compare them side by side.