Enhancement bonuses to armour and natural armour stacking

The Ironskin spell states as follows (emphasis mine):

You gain a +4 enhancement bonus to your existing natural armor bonus (if you do not have a natural armor bonus, you are considered to have an effective natural armor bonus of +0). This enhancement bonus increases by 1 for every 4 caster levels above 4th, to a maximum of +7 at 15th level.

I have a Full Plate armour with a +5 enhancement bonus, and no dex bonus. At caster level 20, what would my AC be? Specifically, does the enhancement bonus to natural armour stack with the enhancement bonus of the full plate armour?

  1. 10 + 9 (plate) +5 (armour enhancement bonus) +7 (natural armour enhancement bonus) = 31 (with stacking)
  2. 10 + 9 (plate) + 7 (natural armour enhancement bonus) = 26 (without stacking)