Realistically the buff will be between +180 and +204 for 5 or 9 Mastery points in the tree respectively.
For most ST abilities in the game, that's a 54-61 dmg reduction while against AOE attacks it can be as low as 14. Still, very usefull when the number of incoming hits is too high like in orvr. Especially if there is no other TOU buff in the party (e.g. KOTBS aura)
Zealots new crit buff from Lunacy will always be +10% but it does not stack with Boost II or Boost V procs (e.g. Sorc Onslaught or Magus Sov). It will still be great for all the other dps classes though.