(going by manufacturer ISOs and not rated ISOs in terms of when talking about various ranges)
In summary 5D3 vs 5D2:
5D3 has a full 2/3 stop better SNR across most of the range, expanding to a somewhat great than 1 stop advantage at ISO25,600. Considering how close the 5D2 already was to theoretical best, that is a really solid improvement, and realistically just about everything one could have hoped for. I'm really glad it did turn out to be the 100% full 2/3 stop better and not the more or less meaningless 1/4-1/3 stop some had feared.
We got everything that was realistically possible here.
5D3 has better DR than the 5D2 from ISO2000 and up (but sadly not one touch better below ISO2000), although not much in it until ISO3200+.
5D3 has basically the same MP count.
5D3 has better color sensitivity than the 5D2 although it's basically a meaninglessly small improvement until you get to ISO6400 or so and even then it's not all that significant until you get above 12,800 where it does do significanty better in this regard.
The 5D3 has one of the worst metamerism scores I've seen though for daylight, much worse yet than even the 5D2, so it will blend even more shades together than the 5D2 did. However, it trades blue for red so it might actually handle pure reds better than the 5D2 although blues worse, so for reds and red-greens it should do better but for blues and blue greens it should do worse. In some ways I think red and red orange may matter more so in the end perhaps it would be a wash and not actually be any worse, maybe even a touch better?? Really hard to say. It's a very complex business and I don't have enough to go on. Have to think about it more. But overall it is one of the most color-blind of all DSLR it seems. They probably had to cheat to match the top SNR using older sensor technology.
LOW ISO ISO100-200: nothing doing sadly, wow, still can't believe they did nothing here, I had taken it as a given it would be so much better here prior to the first samples
(The 2/3 better SNR perhaps mean you can sharpen it a bit more aggressively though? might be rather more color-blind though? maybe a touch less so for red though???)
MID ISO ISO400-800: it's a minor update in image quality
(you should be able to sharpen 800 a bit more without getting noise and I guess 800 now becomes as 400 used to be an ISO where you simply don't worry about noise at all)
HIGH ISO ISO1600+: it is a solid improvement, especially ISO2500+ and a very considerable improvement ISO25,600+
The 5D3 has a better sensor (other than perhaps for certain color blindness, but it's hard issue to sort out), but it doesn't really matter much at all until you are above ISO800 for the most part.
The low ISO is a real let down, not only the same MP (maybe OK if they had to for speed/video) but then nothing with the dynamic range or color? They've just sat around and done nothing. D4 doesn't use any fancy Exmor patents and yet they have greatly improved the low ISO dynamic range on it. The low ISO lack of improvement on the 5D3 is the one area I had been expected a certain improvement and feel most let down about the whole camera.
The high ISO SNR is noticeably better though and tied for best ever.