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the original site with author's comments below. He later said:
"Ralph, I guess I was writing from the ‘is the 6D as good as the 5D III. The standard deviation the 5DIII and 1Dx were in the teens, which is exactly where live view or careful manual focusing is.
The 6D at 29 is actually almost exactly the same as the T4i, so it’s not as good as Live View. But it is better than the 5D II and other older cameras.
And, of course, with most of the lenses in the lineup the difference doesn’t show up, only the newer lenses are capable of the more accurate AF."
To reiterate the above point, look at the standard deviation of AF accuracy in different cameras:
1DsIII 29
5D II 38.5
50D 34
7D 41
1DIV 22
60D 34
T3i 41
1Dx 17
5D III 17
T4i 29
6D 29
The above can be grouped into (based on newly release 28 f/2.8 IS lens):
Tier 1: 1Dx, 5D3... 1D4 trailing slightly
Tier 2: 1Ds3, 6D, T4i/650D... 50D, 60D trailing slightly
Tier 3: 5D2, T3i/600D, 7D