I got my new 5DMKIII a few days ago, but hadn't had the time to test it for ISO perfomance. Finally got to it today.
It looks to me that the new 5DMKIII has a 1 1/3 step ISO advantage over the older 5DMKII. Not the full 2 steps claimed. But as expected, there's always a bit of exaggeration in claimed performance.
I expect the 1Dx to be better... with the smaller chip. Mine's on order. Don't know when it'll be shipped.
Still, the 1 1/3 step improvment is certainly helpful. That extra step of a shorter shutter speed at a sports event in a gym or at a play performace helps.
I could post pictures, but trust me... they'd bear out what I said.