Confirmed 1.9 Patch Notes

[Treesh]

No sane Shadow priest would ever wear Devout, Prophecy, or Transcendence for PvP gear.  Most Shadow priests who want a tier 0 set for PvP will collect Dreadmist for the focus on stamina and the minor Shadow damage boost.  Any Shadow priest who wants a tier 1 or 2 epic set for PvP is SOL, unless they feel like collecting things like the Sash of Whispered Secrets.

So much for "The Balance of Light and Shadow," huh?
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But you're looking at PvP vs PvE rather than differences in specs. Even if Aleri wanted to PvP, she'd still have the same issues that a shadow priest would with those itemizations, despite being holy specced.