Judging by the 1D X samples, the high ISO performance is outstanding and a true 1 stop improvement over the 5D Mark III. Something that strikes me though is 4 Megapixels can't make that much difference.
Canon opted to put a nice new focus system in the 5D Mark III, but it seems to avoid people not buying the 1D X and pro's opting for a Mark III instead, they limited the ISO performance boost. With a boost there wouldn't be much between them but now the 1D X is more appealing with its ISO quality jump!!
The 1DX sensor is an absolute revelation at high ISO. Looking at the flower comparison shots, to beat the 5D3 by a solid 2/3 stops is just insane. This is the all-time low light king ever and it's got to be absolutely at the complete theoretical limit for what is even possible from this sort of design. I'm stunned.
OTOH you did bring up a good point. Sadly, it seems what one connected-one said is true. He said months ago something about how we'd see that Canon marketing thought that they could leave the sensor sort of the same since they were giving us so much in terms of the body specs and that they saved the new process for the 1DX sensor only and wanted a key 5D3 design goal to make it the highest profit per copy yet for a 5 series body. I guess the D800 sensor taught them.
Not that the 5D3 sensor is bad at high ISO though, it's one of the best. But the 1DX is insane. Wow, I mean it's hard to absolutely be sure from that sort of test, it's never more than a rough guess, but I think it's pretty safe to say that at worst the 1DX is as good as any other DSLR at high ISO and it's likely the best in history and perhaps so near the limit that nothing of any sort of even vaguely standard design will beat it.
I wonder how the 1DX will fair at low ISO though. How will the max DR be?
That is where the 5D3 really looks old school.