Dsawemd
Okay, double damage, I haven't actually been ON the server for quite some time, so I don't really renember the zombie damage quite that well. I'll just quickly check up on it.
Okay, so according to the wiki zombies do 90 damage. No biggie. If you doubled that, it would be 180,
MultiHeartGold mentioned a zombie pig did 3 hearts with full armor on. Zombie Pigmen deal 300 damage, so zombies would deal over half of 3 hearts to a max-leveled specced ninja. 1.8 heart, I'd assume. Let's look at it from the low leveled warrior (the toughest unspecced), let's say, a level 7 warrior.
Warriors start with 420 HP and get 9 every level. That'd be 474 hp. 3 hits from "Cannibal Zombie", and he's a goner.
EDIT: Ofcourse, that's if he has no armor on, which would be pretty normal at level 7.
If you were to empower zombies with rotten flesh, I'd have it deal at maximum 30 damage extra. Perhaps more, because it's quite rare, unless ofcourse you throw your flesh at it.
Though I am NOT an expert on buffing stuff.
Whew, my biggest WOT so far!
EDIT2: Ofcourse, that was from a low leveled character's POV, but the mastered ninja would still get blammed after 8 hits, which would ofcourse need the ninja to be AFK, or very unlucky. (Or plain BAD.) Ofcourse, some of the warrior specs would easily take care of it, but you'd have to look from every class's perspective, imho.
TL;DR: Zombies would still be mean, perhaps only 30 extra damage.