Okay, the Mass Heal spell is currently pretty weak due to its terrible slowness, but it's understandable that the devs had to be careful to avoid it being too strong and becoming the only healing spell you ever need, and for the effectiveness of Cleric healing to triple just by advancing from 16th to 17th level.
But here's a fast and easy change to allow Mass Heal to be more useful, without becoming too strong:
Create multiple versions of Mass Heal, chosen from a popup when you cast the spell
Mass Heal Quick would have the same speed as normal Heal, but works on 2 people (and has the increased level cap).
Mass Heal Regular would be a little slower and hit 4 targets.
Mass Heal Slow works as slowly as the current DDO version, and hits just as many people.
There, that way a player can chose to use the current version for the most hitpoint-per-spellpoint cost effectiveness, or he can pick a weaker version that's responsive enough to use during dynamic combat.