Got to play with one a bit at the shop today, shot some dark frames and pushed them as i usually do to better visualize the pesky noise patterns.
The 6D is considerably improved over the 5d2 and even 5d3. Although the total amount of dark level noise is not so greatly reduced, the subjective annoyance of its patterning is noticeably less.
5d2 could produce some fairly harsh noise lines (hor. & vert) in shadows at 100 ISO, they soften a bit from random noise at 400 ISO and up.
6d, at the same ISO settings, produces far less obviously structured noise lines. THIS IS A GOOD THING! Canon has actually made some significant improvement in this area over the older cameras!
6d's overall noise is more random and uniform, tho still exhibits some fine vertical stripe structures at 400 ISO, similar to 7d's low iso noise but not as severe. Overall there is still some coarse but smooth transition variations in horizontal and vertical axis, almost like large blotchiness. This would not show up on large prints anywhere near as bad as 5d2 or 7d noise stripes.
6d's subjective total noise at 400 iso is similar to 5d2 at 100 iso. And the 6d's noise would clean up better with NR software.
6d's high iso performance is impressive!
I could print a decent 6x9" from a reasonably exposed 25600 iso shot! (larger if not too fussy)
So, altho 6D's dynamic range will not be significantly better than 5D bodies at low ISO, its raw files should provide more post-processing leeway because of the reduced severity of patterned noise. Its extra performance at high ISO is a definite bonus.
Fussy shooters who do a lot of post-processing should definitely consider the 6D over either 5D body if low ISO performance is critical to their work.
SoNikon sensored cameras still do blow the 6D into the weeds with far cleaner low ISO data.
Hopefully Canon's (soon to be released?) next generation sensors will further improve on this dark/read noise issue.
As it is now, I am very tempted to sell my 5D2 to fund the 6D, it's that much more usable to me.