If the 1D X is so clean at such high ISO's, I wonder if more people will be able to shoot let's say, night football games with the 300 f/4L vs. the 300 f/2.8L. Now I know the latter will focus faster in lower light, but it really makes you wonder now, if the more expensive lenses like in my example wouldn't necessarily be needed. If you are submitting photos to the AP, the subtle differences in IQ would be irrelevent. Just a thought I had, because I was shooting evening track and field with the 300 f/4L with a 1D4 and I was having no trouble focusing very quickly. However, the ISO performance required a bit more cleanup than I like
I shoot a lot of high school and college football at night, your still going to need that f/2.8 in most instances especially when they get close to the goal line. It is not only the speed of AF that you need to worry about, it is also going to come down to accuracy in low-light conditions. f/2.8 lenses get dual cross type AF sensors so you'll get a better hit ratio. You'll need all the light you can get with teams that have black jerseys, eek.
In my area, the best lit high school football stadiums are actually pretty close to the college stadium (Marshall University in WV) and that leaves me at around 1/640-1/1000 sec at ISO 5000-6400 depending on how close they are to the sideline. At some of the darker stadiums or as the players get down near the goal line the offensive players are often back lit and I've gotta go ISO 8000 just to get 1/400-1/500 sec at time. I prefer a little grain over motion blur most of the time.
This is just another reason why I skipped the 5D3 in favor of the 1DX, I need speed and the cleanest high-ISO images I can get. The 5D3 definitely wasn't two stops better and I would probably still be stuck between ISO 8000-12000 at most. I'd like to get my shutter to at least 1/1000 sec whenever possible.