Well, I've run instances with another buddy that plays a warlock. Problem there is that we run out of mana a LOT more without that 4th VT/weaving/etc. going. For me, it's just easier keeping 4 in sync with a bit of a loss in DPS than trying to micromanage two different priest configs at the same time. We're rarely ever hurting for DPS, and things that wipe us with our current setup would not be any different if I changed one to holy spec. The increase in survivability is not as dramatic as you might think.

If I were to go spend the time getting +1500 healing gear and respec straight holy I'm sure it would help but it wouldn't dramatically alter the outcome of our dungeon runs. Too many other factors make that change a rather small part of the whole picture.