Mickse and myself run this setup, but we're not sure to what extent the Replenishment/vampiric touch-change will actually mean to us.
It's a blast up to level 60 atleast, i've done several instances and as the more talents you get, the more fun it is.
Tanking multiple targets can be a bit stressing, as you need a little time to ramp up the VE heals and stuff, and using shield reduce all tanking except form concencrate, so a lot
of the time I had 1 priest out of shadowform to help a bit on the healing.
As you can see from my signature, I've switched a priest to healing and subbed 1 spriest for an ele shammy for totempower, I haven't started playing this conig yet, but most likely I will start tonight. I won't spec healing until I'm going into instance tho (Ramparts) after I've geared up from the Hellfire quests.
Summary: for bossfights, 4 shadowpriests are more than enough healing, with multiple trash, you might get troubles.