I have used the mkIII for some aerial night photography and it coped well even at 3200 iso. What has surprised me is the 550D I bought to mount on a UAV, I use it all the time next to the mkIII and I cannot tell the difference. In fact earlier this week I was doing some sunrise aerial work, the mkIII was struggling to focus the 70 - 200 mm, I swapped it onto the 550D and no problem. I started with both at 800 iso and noise it not a problem with either. I am staggered and surprised that the little 550D matches the mkIII everytime I shoot them side by side. The mkIV will have to have something seriously good about it to get me to part with more money on the new flagship model, more pixels will not be enough.
Kevin.