Mastery from prot increases critical block chance, with like 1/3rd of that amount increasing AP
As of now i got around 53% mastery infight with 721 pvp ilvl. That results in 51% critical block chance and 34% attackpower bonus. so its rather 2/3rd i'd say.
As normal prot w/ anger management I out damage every single class in the game. Maybe warlocks will out damage me with the haunt buff, but last time I played prot I was clearly ontop by at least a million each game. So much sustained damage it's crazy.
I'll give AM a shot, biggest loss for me for going with Glad Stance is the offensive dispel that you miss out on. But as i said, i just gave it a try and its pretty fun.
EDIT: Just specced away from gladstance and to AM + SD. And it feels pretty consistent. Even though i do miss the Burst of Shieldcharge and Heavy Repercussions. If you haven't played it for a longer time, maybe give it a try. It just feels more bursty, i'll stick with it as long as im not going for rating anyway.