Wouldn't Cartel Aristocrat fit pretty well into this deck? Human, sac outlet, and works well with servitude.
Isn't Banisher Priest kind of a non-bo with Drown in Sorrow? I understand you're trying to keep the mana costs down, but going up the curve a bit, have you thought about adding Dictate of Erebos to make sure Drown in Sorrow takes out their whole board (it's evil, really, Drown will kill your stuff and their stuff, and then when their little stuff is good and dead, Dictate comes through and takes out anything that Drown missed, like Stormbreath Dragon or Blood Baron of Vizkopa). Since you're already running Whip of Erebos, I'd find it hard to not run Obzedat, Ghost Council as a 2-of. Also, Dictate of Heliod could be a surprise blowout if you wanted to go the "overwhelm in combat" direction.
Why not run fiendslayer paladin instead of banisher priest? same cost, both human, difference is first strike and life link vs. spot removal.