Having looked at the 1DX2 specs over at NL it says:
Dual DIGIC6 + processor
ISO100-51200 (extended with 409600)
I can imagine that a faster processor and on chip A/D on a new sensor will enhance IQ at ISO 51k quite a bit.
As I do not know how many times a PJ is forced to shoot at these ISOs, I have no idea how IQ looks at the moment in the 1DX. But I think, Canon better deliever solid ISO 51k instead of a zillion ISOs as noisy as an old B-52
Here's a beta D5 field test (maybe you read it already) where the photographer gets to the verdict:
For low resolution usage, magazines and newspapers I find images exposed up to ISO 51200 usable, with still not much noise in the shadows or colors. At ISO 102 400 the color noise starts to be too disturbing in my opinion, but with some noise reduction plugin / software this can be fixed (to some point). Above ISO 51 200 – 102 400 the noise is visible with reduced color quality, sharpness and the overall image quality and I’m not uploading any images at higher ISO value than 102 400. At maximum ISO H5 (= ISO 3 280 000) the image quality is poor, but at least you can see (with a lot of color noise) what you have been shooting.
http://oleliodden.com/photo-gear/field-reviews/beta-test-report-nikon-d5/