“ Beyond all that, keep in mind that all AF engines work better at larger focal ratios. So if you have an f/5.6 sensor it will work better at f/4 than f/5.6. By extension, an f/8 sensor should work an f/5.6 configuration with more accuracy than an f/5.6 sensor because it is working well within its spec, and not at the edge at the absolute wall.”
I have seen this already with my D800 using the TC20E III on the 400/2.8. This combination results in a max aperture of f/5.6 but on the D300s the lens focus slowly. On the D800 the lens snaps into focus.
Also, the f/8 capable sensors will continue to operate in low light situations when the normal AF sensors begin to fail regardless of the lens used.