Hypersmurf
Moderatarrrrh...
You're trying too hard to make it look absurd. The described limit is within the context of the spell.
No, I'm not, and it's not stated as having anything to do with the spell. It states that armor cannot have a bonus higher than +5, whatever the source.
The location of a rule does not limit its scope. To believe otherwise means that the Two-Weapon Fighting feat reduces the penalties for throwing axes with both hands, or that enhancement bonuses on bows provide no extra hardness, extra hit points, or imunity to weapons of lesser enhancement.
Either the phrase is meant to include the base armor bonus of a suit of armor, in which case heavy armor does not exist; or it doesn't, in which case Magic Vestment can give a mundane suit of full plate a +5 bonus.
Since it is quite clear that heavy armor exists, the second case must be true - the passage in question does not take into account the physical armor bonus provided by the suit of armor, only modifiers thereto.
-Hyp.