Hi All,
Remember these are each 25% of your army.
In my opinion, they will have to be significantly better than that to see any play.
I would expect them to be able to kill a 30 wound unit with a 4+ save and no after save on an average roll. (and assuming top profile) and they will also need to be able to survive a round or two in combat. I would put them in them in the ballpark of 30 wounds with a or 5+ save or 25 wounds with a 4+ save. Hopefully they'll also have a mechanic with a built in -1 to wound them so they might live through 1 round of fighting.
This is pure speculation but, GW will need rules to make sure everyone wants to buy the kit. Especially, since everyone can use one.