I rented the camera and lens, curious what could be pulled out at night, then took it to the SD Badlands. My initial feelings as far as noise was that it wasn't any better than the 5D II when I had it. Turns out I guess it is a little better. I tried to compare some with the 5D II images I have, but obviously I couldn't do same scene comparisons since I don't own the camera anymore. Had the RAWs converted to DNG then opened in LR3.
Anyway, a bunch of images there with some comparison crops on the second page. Also on the Milky Way images, I really didn't do any noise reduction(except twilight one and not on stars). They could be cleaned up and look quite nice, but I really don't think you'd even need to do anything but color noise removal to the sky/stars/Milky Way even up to 8,000 - 10,000 ISO I guess.
In the end I don't know that image quality would be worth an upgrade from the 5D II but perhaps up at the real high ISOs it may indeed be worthy. I never cared about speed or focusing, or even video, so I didn't pay all that much attention. What is rather cool is that live view leveling. So nice for dark night shooting.