As a long time (Almost as long as it has been out) 5D2, owner and user, and some one who loves to push the high ISO settings, i can assure you that the 1DX is far far superior to the 5D2 with its high ISO capabilities.
Forget the numbers that a played out, the 1DX has an ISO of 51K that certainly matches the 5D2 at 6400 if not 3200.
Really - they showed a print of a scene that i have an almost identical version of taken on my old A1 with 100asa film, a tripod, cable release and several seconds of my time.
I took shots with the 1DX and compared the same shot with my 5D2 - oh no i didn't, because my 5D2 couldn't work in such low light, nor could it focus in such low light. The 1DX really is on a different planet from the 5D2 where low light is concerned, and my 5D2's frequently see 6400 (even in daylight) and are no strangers to 25600, which always needs a bit of treatment.
I fully expect the 5D3 to make 12800 a working ISO, if not 25600...