OK,
1DX II is the only full frame high fps sport camera that can detect flicker and for someone shooting in dark high school gym like you and I, you should value that very much.
If you are like me and that the only shots that really matter are the ones where the puck is about to leave the gloves of the goalie, or the punch just broke the jaw then you would appreciate the much better auto focus of the 1DX II. It is way snappier, it holds that focus much longer and regain it faster, which in turn allows you to get closer to 12 fps, while on the original good luck in having 7 or 8fps.
Back to the high ISO, I agree with you, me too I don't see much of improvement especially in the mid tones quantity wise, again shooting in dark high school gyms. These people claiming 1 stop improvements shoot outdoor or in well lit labs.
However (however, however, however) the 1DX II detains remarkably more detail and sharpness in the "in focus" portion of the photos. Bring your photos to Photoshop and do your standard post, then create a composite layer (Ctl+Shift+Alt+E), apply a noise reduction until you like the noise level at the background, create a mask and paint away the noise reduction on the in focus portion of the photo, compare 3200 from the 1DX vs 6400 and 12800 from the II and see for yourself the difference. Don't take my word and don't expect any photo from me either.
Not the best solution for JPEG shooter who had to deliver the photo right away, but if you have the time to post, the II has more latitude left for you to make it a keeper.
Just my 2 cents.
I am very happy with mine, especially that I bought it for the DPAF on video.