A 1DX is a professional full frame body. A 7D MkII is a semi pro APS-C body. The difference in camera operation, build quality and high ISO image quality will be huge. However, if you're doing a lot of wildlife and BIF photography then factor in that you will lose a lot of effective reach with your lenses making this switch. The 7D MkII has a 1.6x crop factor due to the smaller sensor
I use the 1DX, 7D2 and 100-400 Mk2 - not much experience with the Sigma.
For outright AF performance, IQ, performance in adverse conditions and responsiveness the 1DX is the better camera. Yes the 1DX will give less "Reach" but this only happens under ideal conditions - for everything else the 1DX is just a better camera IMO.
Before you decide have a play with a 1 series first - they operate a little differently (better) but they weigh a ton!
It's not just a little bit - with a still subject filling the frame then you might not notice a massive difference - for anything moving then the focus and image quality are loads better with the 1dx - I've got two and use them to shoot sport with a 400 f2.8 and 70-200 f2.8 - but I also use them to shoot wildlife as a hobby and they are great with the 100-400 mkii - it also plays nicely with the 1.4 and 2x convertors
Just bought a 1DX myself a few weeks ago and I am afraid the 7D MKII will sit in the bag for a while. Just shot my sons hurling match last night (finished around 9pm) no floodlights came on ,and with 1000ISO it rarely missed a beat.
I almost exclusively shoot wildlife (and steam locomotives when given the opportunity) and had a 7D-2 for several years, upgrading from my first digital camera, the excellent 70D. I later bought a 5D-4 as a second body and my first full-frame and found that I used it more than the 7D-2. A year later I had the opportunity to buy a brand new unused 1DX-2 with a substantial discount helped further by selling my 5D-4.
I was always very pleased with the results I could get on my 7D-2 but was hardly ever using it once I had the 1DX-2 (the 1DX-1 is very similar I understand) and so I sold it together with my excellent but too small EOS M5 and replaced it with a EOS-R to live mostly on my Macro lens.
So to answer the original question and to confirm what others have already said, the 1DX is a different league. I would add that if you use big telephoto lenses the 1DX is better balanced physically although mounting a Canon battery grip on the 7D-2 helps. On a tripod it doesn't matter so much.