I don't have any experience 5-boxing them but a pally buddy runs with my 4-SP team for instances when we're both on. Did you have anything in particular you wanted to know?

If I can remember, I'll post up some vids of us doing some bosses in Outlands stuff. I did not quite have what it took with just the 4 priests to kill stuff like Ruul the Darkener, the Zuluhed quest and those ones in the Path of Conquest. With him along, it completely trivializes the content. Movement is a bit of a problem (as is normal with multiboxing) but no big deal. We've done Ramparts, Blood Furnace, Underbog and some of Steamvault. It gets rougher trying to not pull aggro with all the VE/renew spam but as long as he keeps aggro, fights are fairly easy. The hardest fight we had to date is Swamplord Musel'lek in Underbog. His AoE traps were rough.

The hardest thing in playing with the pally is getting the full duration of Vampiric Touch so you make back in mana what you used to cast it.