here is what happening" since 2008" sensor and todays 6D sensor
improved QE, higher FWC, but still a lot of read out noise and low DR at base iso,
improved high iso, and less banding at base iso
however, the slight reduction in 6D's read noise figure does not tell the whole story.
We don't know HOW the read noise is calculated. To me it seems they're taking an AVERAGE read noise value instead of a peak-to-peak read noise value.
The difference being you can have 2 levels of average read noise that are very similar, as in the 5d2 and 6d, but they can have different peak-to-peak values.
The peak-to-peak read noise levels are more relevant because the peaks
are what become more visible in shadow areas.
In the case of the 5d2 the peak read noise occurs repetitively when going across the sensor horizontally, which results in a pattern of vertical noise stripes visible at lower ISO settings. These stripes get obscurred by larger amounts of relative random noise as you increase ISO.
The 6d's low ISO readout noise is much more uniform from pixel to pixel, resulting in a smoother looking tone in the shadow areas. This sort of noise also responds better to NR software so you lose less actual image detail because you don't need to smear the image so much to obscure the banding structures.
In this respect alone, the 6D is quite far improved over the 5d2 and even the 5d3.
As an overall package, I'd still consider the other improvements and refinements of the 6D over the 5d2 as significant.
I'm selling my 5d2 to fund a 6d. The 6d is Canon's best IQ per cost FF camera and if you don't need the performance features of the 5d3 or 1dx it's a no-brainer.
I don't care about video so haven't compared it.