I'm currently leveling a Pally/4 Priest team and it's loads of fun. My Pally is 80 and the Priests are 78/74/74/73, but I can easily handle DTK runs in ~30 mins (still farming gear) and have had no trouble with most instances. On single targets, they pump out nearly 1K dps each. With a group of 5-8 using Holy Nova, I get closer to 2K. Will they be viable at 80? Not sure, but I've been pleasantly surprised so far.