On the 25 man the back two hit for a whole lot more then on 10. We ended up having 2 tanks (a feral and a warrior) on them caus our mages were just taking too mutch dmg, just shoot them once and you're set. The adds ofcourse won't take any dmg and thus the fight will last a bit longer, but it's a lot more controlable since one hit won't take away 50% of your health like happens on a mage.

My guild's run this with three priests before, but we've thus far felt that having the third priest doing normal priestly things (healing or DPS) is more beneficial than just hanging around with the extra Understudy locked down.

It might help to train a greater variety of priests with what they'll need to do to tank this, should the usual priest tanks not be available on a given evening, but other wise I'm not really sure that MCing 3 provides enough of a benefit to yank the third priest from their usual role.

That said, it might be a good strategy on nights when the Naxx lag is particularly cranky. Delayed cast time, more time wasted between a landed MC and actually being able to control the NPC, would certainly shift the balance in favor of keeping an extra Understudy primed to take up aggro. You'd just need to make sure the MC is a bit staggered, so they're not all dropping at the same time, leaving you with nothing MC'd and the raid SOL.