Realistically, I think the 70-300mm L will do fine with the horizontal line points - the DoF with an f/4-5.6 lens is pretty deep, the more accurate sensors are primarily beneficial with the thinner DoF of faster lenses.
The dof might be not razor thin at tele range, but that's no substitute for pattern detection as you know. But you don't have the 70-300L, so I guess you cannot tell how good it really is.
Thanks for telling me about the groups, just looked it up in the 5d3 manual an this isn't specified in any other 1dx/5d3 af reviews I looked at. Even the 180mm macro is a "Group G". Is this Canon marketing at work, or is this due to some real hardware considerations?
* The 24-70 is Group B, while the older 28-70 is Group A???
* Both the oldest 100mm non-L *non-USM* macro and the new 100L macro are listed as Group C, while the 100mm non-L *USM* is only "Group E" without cross-point sensors to the side. And why isn't the 100mm L macro "Group A" like the 70-200/2.8?
* What about 3rd party lenses like the Tamron 24-70: What group rating do they get?
* What about the Kenko extenders, do they impact the group rating like the Canon extenders do?