PvP Shadow is a different build than the shadow/disc build for leveling. First, it does not have improved fade, and generally it has mind blast maxed out. Second, Spirit Tap is swapped for Blackout.  This is the build if you want to head into Alterac Valley by yourself, or just roam around looking for trouble. As I mentioned above, using Flay puts a big ol "Kill Me" sign on your head, so you tend to lean more on the less mana efficient (though crit capable) Mind Blast.  Much of the time, people that play this build have a set of with +shadow damage items.  It is not unlikely for a priest with this build and good Tier 1 gear to crit a mind blast for 1500 damage and deal 300 DPS until they run out of mana (very quickly). A good shadow priest can win 2 on 1’s against some pairs without healers.
 

The disadvantages of this build are just as extreme. While a Shadow spec priest can heal in end game instances, it is more difficult. They lack the mana efficiency and healing power of Discipline and Holy priests.  Most groups will accept a shadow priest, but prefer a holy one.  Shadow Priests are also very ineffective in dealing damage to against elite mobs. These mobs tend to have more health than the priest has mana and shadow priests do not have the agro control to keep the mobs off of them.