You'll see MUCH bigger variation out in the field when you're using phase-AF, and even then you'll probably hardly ever notice it in a real shooting situation.
Besides, apertures can vary alot, and there is VERY few lenses that follow the "soft open, better one stop, very good two stop down and diffraction softens at f22"- curve. For example the 50 L is sharper at 1,8 than at 2,2.
And the 24-70 mk2 is no different and it still is the best zoom ever, but it's when a test charts shows you a slight difference it's a bad copy and it sucks and Canon should burn in hell.
I like my lenses to be optimized as well, and I have had several copies of every lens (never saw a difference) and I adjust them with AFMA, now lately, with the best method, FoCal. They are adjusted as good as they can be on the best AF-body ever made, and that's good enough for me.
Perfect phase-AF is a myth.http://www.lensrentals.com/blog/2012/07/autofocus-reality-part-1-center-point-single-shot-accuracy