I just took delivery of mine, and whilst it looks to be an awesome camera I really can't say I'm impressed with the ISO improvements over the 1Ds3.
Generally I never shoot above ISO3200...not because I don't want to, but because of the noise level. I was hoping the 5D3 would be of some improvement over the 1Ds3 and in a way it is.
Silent shooting really is almost silent for example. But with all the noise reduction disabled I don't consider the 5D3 to be even 1 stop better in noise handling over the 1ds3:
Note also that the 1Ds3 technically only goes to 1600, the 3200 option is a 'H' mode.
These are all from in camera Jpegs too, to rule out the Raw convertor.
However, the files are much more robust and maleable.