This is a good summary of all abilities - https://www.youtube.com/watch?v=hdo3JdYgQGU
Play action most likely improves on a straight line between 50 and 99 like most other abilities. Whatever effect it has will show little improvement between 85 and 99.
Playaction only helps defenders bite down on pump fakes. But it does nothing if the person pass commits
I see GT Kelce only has 89 play action. How much will this affect him? Don't plan on using him, but guess this means the pa boot over cheesers will stay away from him.