I disagree.
There are enough other drawbacks to necrotic touch, not the least of which is that it's self-only for all intents and purposes. But mainly, because Favored souls have big old quickened heals for when things get really rough. Pale Masters wouldn't be able to sit in groups of enemies keeping themselves up in nearly the same capacity.
It would probably be overpowered at level 12, and I would have no problems adding it at level 18, where it would already be balanced.