Ok, I know we're talking APS-C here, but have you ever shot at ISO 3200 and ISO 6400 with the 1D Mark IV? Completely useable, if not underexposed. I think this more speaks to Canon's flagship line manufacturing though, vs. the general category of APS-H sensors. I have a lot of night football game shots at ISO 3200 with that camera that are probably not much different than the 1Dx shots at that ISO, same situation. This is where I think a 1D4 with 300 f/2.8 might be a better choice than a 1Dx 300 f/2.8 but with cropping in post. I could be wrong.