Okay, that makes sense. However, these were field shots at a soccer game under lights and ISO 25,600 is so grainy and noisy that when you apply sufficient NR, the faces and soccer ball are blurry, which I cannot have. That's why I was saying in this case that the photos were unusable. I suppose when you have more, even lighting it is different. The shadows are actually really, really bad. I think the problem is that this "ISO advantage" is based more upon specs and even lit situations, vs. real-life harsh situations.
I'm curious what the full camera settings were. I've browsed through quite a few sample shots from the 1D X at high ISO, and what your describing is rather surprising. The only thing that I can think of is that you were shooting at REALLY high shutter speeds, and were simply not getting enough light down the lens. And, if that were the case, I would actually recommend using ISO 51200 for better results. If you can't get enough light down the lens, a higher ISO setting, even if its the maximum, will almost always be better than shooting with the wrong settings, as photon shot noise will completely dominate. Also, keep in mind, "noise" at ISO settings as high as this is all relative. The noise on the 1D X at ISO 25600 is only going to be "relatively less" than noise on an older Canon one to two stops lower. For example, the noise on the 1D X at 25600 is probably similar to noise on the 5D II or 1Ds III at ISO 6400...thats still a lot of noise, its just that relatively speaking, its lower than the noise would be if either of those two cameras were actually capable of ISO 25600.
That's exactly it. I was shooting at 1/640s and for soccer, that was still a bit slow. Next time I will accept the low light and open up the aperture, and just realize that I'll have to go for specific player shots, as DOF will be limited. I was 1/640s at f/6.3, ISO 25,600. In the future, when it gets that dark, I'll use wider aperture values, and I'm not afraid to go all the way to f/2.8. The point is, in my kitchen shooting at ISO 25,600 I could clean up, but out in real life on the field, it didn't work out so nicely.
In your kitchen, your subjects are probably only a few feet away. Out on a soccer field, your subjects are probably ten times as far away. Remember, light has an inverse square falloff...even if its a bright light source, on the field that light has to travel from the bulb to the subject, then from the subject to the camera. Our eyes adjust automatically, however cameras are fixed devices. A soccer field is not a particularly bright place in the grand scheme of things. The difference between f/2.8 and f/6.3 is more than two stops, or more than a four-fold difference in light let down the lens. If you simply double your aperture to f/4.5, that alone would probably do wonders (either you could drop to ISO 12800 and get more DR, or keep using 25600 and get a more saturated shot.)
I agree that all of this is true. Which simply bolsters my point about shooting night soccer at ISO 25600 with a 1DX. It's beyond lab tests and spec sheets.