Neuro, don't you mean 'vertical' sensor line (for the 6D's centre AF point?)
And am I correct in believing that AF points with vertical sensor lines pick up actual horizontal contrast in scenes easier, eg the actual horizon? (and visa versa, that horizontal sensor lines pick up vertical lines easier, eg the sides of buildings?
No, I mean a horizontal f/2.8 sensor line for the 6D center point.
But yes, a horizontal sensor line is sensitive to vertical lines in the scene, like the side of a door frame or building (and vice versa).
When you wrote "It doesn't matter whether the lens has an f/2.8 or an f/1.0 max aperture, the AF system will be precise to that degree relative to the max aperture of the lens."... does that mean that the AF system will or will not have more precision with a f/1.0 lens than a f/2.8 lens?
I do understand lenses are held 'wide open' until they are actually stopped down when taking the photo [or when the depth of field preview button is used.]
Same relative precision, but in an absolute sense, an f/1.0 lens should be more precise. Honestly, I'm not absolutely certain of this myself - in this case, I'm basing this on an explicit statement from Chuck Westfall.
And finally, Neuro... can you define both precision and accuracy... comparing / contrasting the two?
Precision is repeatability, how tightly a series of repeated measurements cluster together. Accuracy is how close the average of a repeated series of measurements, or even a single measurement for that matter, is to the true value.