mass confusion
It seems that there is some confusion here and several people are arguing about completely different things with regard to the max +5 issue. I'd like to try and help get everyone on the same page.
There are two issues here, the normal armor rules (+5 max enhancement, +10 max enchanment) and the ambiguous comment at the end of the spell.
The normal rules don't have much place here, and no one is contesting them. The people above who mention them seem to be just confused about other people talking about the spell.
The comment at the end of the spell about +5 max bonus is definitely ambiguous and can be interpreted several ways. I could be a very poorly written reminder of the normal rules, but it would be very poorly written in this case. However this is possible, and it might have said more initially and got a bit garbled during editing.
The other interpretation is more along the lines of the suggestions above, that it is a max AC period for use with this spell ... in other words this spell as NO EFFECT at all on full plate. That is not to say you can't have magic full plate, just that this spell doesn't work on it.
The wording does seem to imply the second option over the first, but it is badly written for that option too, but at least it's a bit closer.
I tried comparing the spell to Greater Magic Weapon to see if that would help give some clues. They both have the same duration, so if you look at it "financially" (how much you save over have to make +5 armor) it seems fine for the first interpretation ... magic weapons cost more than magic armor with the same bonus. However, there is no similar comment at the end of magic weapon or Greater Magic Weapon, so that implies that second option since if it was just a badly worded reminder you would think they would have the same under the weapon spells.
My 2 cents anyways.