From some limited testing with a 20th level divine caster it seems that mass heal doesn't cap out at 150 as per the description which means that splashing will affect mass heal quite a lot.
Probably best if this gets confirmed.
I love this idea of an "advanced" cleric, other people call it a hybrid build, melee, offensive casting and healing but I'm not convinced that it is viable on a pure cleric. Surely a monk splash brings so much to the table? (+1 dc, feats, skills etc).