Ok, I'm not completely up to speed on this, but considering my love of playing clerics, couldn't you simply set the range on this to say 5 feet? And then possibly increase that range every other level (1, 3, 5, 7, etc...), or even more drawn out, make it every 3 levels (eg: 1, 4, 7, 10, etc...). This would reduce the area that a priest could effect with this. Granted that a high level priest could still effect a large area with negative or positive energy, but it is a bit restrictive.
On a side note, this honestly sounds like some MMO maneuver, I mean after a while priests get mass heal, or mass harm anyway, why change that?
- Jesthik