I was using 15223 for the magic number, which I got from http://www.wowwiki.com/Formulas:Damage_reduction - if their numbers are wrong, please update that page with the right ones. Also, remember that we're discussing boss armour against an attack from a level 80 and not the usual discussion of a player's armour against a level 83 attack - use the level 80 magic number!

100% ArmPen isn't realistic, I was just demonstrating the extreme case. We can talk about cases that'll actually come up...

The 20% ArP from a 5-sunder leaves a Naxx boss on 11101.8, which is 42.16% damage reduction = 57.84% damage taken which comes out as a 7.5% increase over zero ArP. A mace-spec arms warrior in battle stance hitting an unsundered boss has 25% ArP (assuming ArP is an addition-stacker, not a multiplication-stacker), leaving the boss at 10606.5, or 58.94% damage taken which is a 9.59% damage increase over no ArP. The 45% from a mace-arms warrior beating up a 5-sundered boss has the boss on an effective 8625.3 armour for 63.83% damage taken, which is an 18.7% increase.