Two stops of ISO? I don't think that's theoretically possible. 1/2 stop of ISO is a big difference.
The difference between the original 5D and the 5D MK III is less than 1 stop. The original 5D is the lowest scoring Canon FF sensor (as far as high ISO is concerned) that DXO has measured.
Even the 1D X is a hair less than 1 stop better than the original 5D.
I would disagree with this. the 5DIII is one full stop better then the 5DII. (Shooting RAW) Testing has shown this as well as my real world experience. The 5DIII is at least 2 stops better then the 5DC.
I'd be happy to see some reliable test results other than those of DXO. DXO has a pretty good and accurate test capability. What many of us do not like is their self assigned sensor ratings which reflects some unknown weightings that may or may not reflect our actual usage. The accuracy of their basic test results though is well accepted.
I've had the original 5D, two 5D MK II's and two 5D MK III's. and I'd love to be able to say my MK III's are full stop better than my MK II's. But that's only wishful thinking.
There are well done accurate measurements at DXO of the high ISO capability of various sensors, the range is much narrower than people think.
Here is their high ISO rating of CMOS Sensors RAW output since 2004. I did not include the CCD sensor from the original 1DS MK I, (ISO 954) because it was a different technology.
From the lowest to the highest CMOS FF sensor, the difference is about one stop.
EOS 5D - ISO 1368
EOS 1Ds MK II - ISO 1480
EOS 1DS MK III - ISO 1663
EOS 5D MK II - ISO 1815
EOS 5D MK III - ISO 2293
EOS 1D X - ISO 2786
EOS 6D - ISO 2340
I as well have owned these three generations of cameras and my opinion is based solely on real world experience. I really don't hold much stock in tests that DXO or any other outfit performs. They are a nice comparison on paper but once you get out into the field that "testing" doesn't always hold true.
Perhaps I should elaborate on my statement. I believe that the 5DIII is a full stop better then the 5DII regarding noise for a couple of reasons. The type of noise between these two cameras is very different. The 5DII has colorful noise which in my opinion eats up detail and is not that easy to eliminate in post without even more detail loss. The 5DIII has strictly luminance noise (in the usable iSO range) which is much more easy to eliminate in post and the detail retention is much much better. Images shot at ISO 6400 with the 5DIII are often better out of camera then ISO 3200 of the 5DII. Then because the noise is easier to manage there is even a larger divergence.
I find that I rarely apply NR on images shot at ISO 3200 or below. It almost looks to me as if the luminance noise of the 5DIII resembles sharpening artifacts. Either way I'm confident shooting at 1-2 stops higer in ISO with my 5DIII then my 5dII. Hopefully this explanation helps clarify where I am coming from with my comments. Happy shooting.