I would like to add there is a long standing Prot PvP guide that was made last season that has a ton of great info in it. Most of the top rated Prots have contributed some info in there. Here is just a quick snippet about Mastery in there.
I recommend checking it out for ideas and feedback to improve. As the season goes on sure more people will comment about new builds, Leggos and what they are having success with in PvP/RBGs. Good luck and have fun
The 40% was changed to 50% increased dmg for Tanks in a hotfix which is why he said 40%. Hope this gets revisited and taken back down some.