ah guys befor i forgt this one when you pvp , set the Refresh Time on dot power increase to zero 0 , its not needed to apply new dots when some stuff proccs like trinkets etc , but if you play pve musst change it like in the PvE.xml file again, in pvp ist all about the proccs and how fast you get it and hit the target hard with it , and go for int > haste at ( 8085) > crit > mastery >. for pvp dots its ok to take 3x3 or 4x4 VT and sw: p 100% 51% mana , 3x2 when mana between 50% and 21% , and 3 vt x 1 sw: p less than 20% , best talent to take for rbg bgs stuff is from darkness comes light . enbale targeting in the CR
take a look here more from the best shadow priest in the world Talabdar this well help you more too
Shadow Priest PvP DPS Guide for MoP
https://www.youtube.com/watch?v=E7UqAMAbsXI
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Shadow Priest Guide - Mastery vs Crit! - Mop 5.4 PVP
https://www.youtube.com/watch?v=TWSsThxYo10