Phase detection AF improves tracking dramatically, as demonstrated by the A6000. My guess is the A7s mkII will have the upgraded AF. The single shot AF is really good, but its no speed demon.
Keep in mind this was done in low light with low contrast, the subject was wearing dark clothing with no contrast. I just did a test on a bright yellow object with a grey-white background with tons of contrast and the A7S locked on 80% of the time.
So the -4EV claim by Sony is only for Single AF! And yes we need phase detection! I still think Canon's dual pixel AF is far superior to what Sony has right now.
