Chill

Pick Dwarf or Undead. Edit: Nvm. You're going UD. For leveling Go 5/5 Spirit Tap then 5/5 Wand Spec until 40 then respec Shadow Form until 50 then respec Disc/Holy.

In other words, "Buff group rewards. Nerf solo rewards." No comment.

Frankly, I agree with Res on his points. You aren't going to be getting loot over Mages/Warlocks at all. It doesn't matter whether it's BiS or not. Also, Support Priest is flat out better for raids than Shadow Priest. That's a big statement I guess. I don't have proof on hand. But, I can bet you that a little bit of theorycrafting will make it easy to understand. Bottom line is, Shadow Priest is for Leveling. If you really want to Honor Grind, grab Silence, Mental Agility, Mental Strength and Healing Focus then go spot heal a Warrior. Shadow is great if you're already geared for it. Do you believe you're going to have gear for it any time soon?

People like to be lazy farming old content. This doesn't make the cut imo. If you want to do something weird, start with core. A Tri-Spec with Shadow Weaving is good enough to see progression, so use that instead. PI isn't good enough to compare to core. It just isn't. 14/17/20 should be the build of choice. Those are the magic numbers for a SW Support Priest. Go 4/5 Holy Specialization, 3/3 Inspiration and 2/3 Improved Healing. Don't skip Inspiration. It's too useful to be passed up for other things.