Few things:

You don't need spirit to get hit capped. There's a glyph which increases smite's hitchance by 18%.

4 disc priest is extreme overkill, if you overgear the instance a bit then 1 priest will keep your tank up pretty well. Especially if you throw in a renew.

So I really don't want to burst your bubble, but I don't think it's that effective.

My priest sits at itemlvl 353. Quite geared, no upgrades in HC's anymore and pushes out 6k max with a lot of luck and Smite hit glyph and holy fire glyph(+20% dmg to smite if holy fire dot is present).

I would allways sub out 2 or 3 priests for arcane mages because they're so extremely OP to box, because they have the easiest rotation ever.