I'm going to assume by your results that.

Mnd = Healing potency
Pie = Healing cap

Currently heal spells have a "Hard cap" that they wont go over, possibly determined by a players Piety or certain skills "Prime conditioning"

This can easilly be seen with skills like Bloodbath II that drain 100% of your damage (To a certain cap) if someone happened to be more energetic than me with a marauder at 50 they could test this easilly. Bloodbath II will only cure so many HP regardless of the damage you inflict (I think it was around 800HP for my character)

Equiping something with + Pie to see if that raises the capped heal rate would be rather easy...but I'm lazy.