For me, I see a 2-stop improvement in ISO performance.
This is why I feel this way:
1.) First off, noise is finer. Detail is higher at all upper-ISO levels. Seriously, ISO 25,600 looks excellent. I would try and avoid above ISO 6400 on the 5D2. The noise on the 5D2 above 6400 got blotchy and hard to clean up, even in LR4.
2.) Contrast and depth of the image at higher ISO is improved. No longer look so flat above ISO 6400 . The images look more pleasing (again, RAW as well... I have played used/edited them in CS5)
3.) The criss-cross banding you got at ISO 12,800 and 25,600 on the 5D2 is gone. It is extremely clean right up to 25,600 on the 5D3, where you may start to get some artifacts in some conditions but generally don't appear until 51,200.
4.) ISO 6400 was my comfortable limit with the 5D2... My comfortable limit with the 5D3 is ISO 25,600... Two stops.
So there is how I got my 2-stop improvement. There is more to it then just noise levels... The quality of the noise, contrast and color rendition of the image at such high ISO... For some it may only be 1-stop (or less), depending on their standards.
I can't wait to see what LR4 will be able to do with these RAW files... What is taking them so long!