My neighbor / cousin-in-law...just last week, bought a 1DX with his newly earned "toy money"...and claims he has now relegated his 5D3 to his wife, with a 50 1.4 attached, to make it easier for her to carry around.
So I hope to attempt to put the 1DX to the test in low light at some point, hopefully with my 135L mounted.
I must say, holding his 1DX with 70-200 f/2.8 series 1, mounted...and then my 6D with 70-200 f/4 (non-IS), it was quite a lot more weight. Holding his was kind of like holding three bricks, mine fealt light as a feather. I would not want to walk around with that weight all day. I own one lens that is a bit more weight than a 70-200 2.8, but I use it on a monopod. I've rented superteles, and used them on the monopod also (or a tripod).
I might be seen as stupid for asking this, but it seems like much of the recent discussion in this thread, has degenerated into speaking about the 1DX's dynamic range and s/n ratio, at lower ISO. And yet, the OP shoots basketball games, etc...in less than ideal light at higher ISO. So why even discuss the low ISO capability, especially if all the independent tests indicate that higher ISO is where the 1DX really shines, and was obviously meant to be used?
It seems to me, that rather than compare the 1DX to Nikon crop bodies at low ISO, or to the 5D3 at most any ISO...that it should be compared to the Nikon D3s. I suspect the D3s would give it a serious run for its money, perhaps in the ISO range from 1000 to 25,600. Of course the file dimensions are only 12 MP from the D3s, but it would still be interesting to compare...because if the D3s really does have lower noise of both types, then the actual or practical resolution from the RAW files might be equal or even superior to the 1DX (at least as ISO goes up, perhaps above ISO 4000 or 5000).
It's interesting that the noise pattern of the 1DX might be larger and more difficult to overcome in post editing, since I've found the same true going from the 5D3's files, to my 6D. The 6D's noise, especially the luminance noise (or "grain")...is about 1/3 the size of the 5D3's...at least on these particular bodies that I've used.