The best Shadow priest race is undead, because of their racail priest skill that they get. Devouring plague. its a extra dps spell that does crazy damage, and heals you to top. SW:P causes 1236 shadow damage at rank 10, as devouring plauge 1216 at rank 7, and it heals you.
Also you cant forget about Will of the Forsaken.
SW:D FTW!