Here's a real-world shot at ISO 6400 that I took tonight for a friend with my brand spanking new 5D3. Shot in RAW, no noise reduction added (camera set to Standard, but I don't think that affects RAW at all), 180mm, ISO 6400, 1/50th, and f2.8.
Only edits are lens profile (70-200 2.8L), strong contrast curve and I enhanced her eyes in Lightroom 4.
I know this is higher ISO than you were talking about, but I think it may still be helpful.
Kev
Looks like there is quite a lot of Chroma NR being applied to this picture. ISO 6400 with absolutely no NR looks very messy, but it does clean up quite nicely, like in this picture.
I have shot SxS comparisons between the Mk II and III at all ISOs. I'd say the difference from 50-800 is negligible. At higher ISOs, the amount of (unreduced) noise is actually quite similar, but the Mark III files look much better after NR. IMHO
Even at very low ISO's, if you push the exposure in post, you will see ugly blotches of color noise in the shadow areas in Mark II images. Mark III files have the same issue, but it's slightly less prominent.