AP appears to apply to a shield strike, as if it were any other offhand weapon. There is a bit of inconsistency with the AP on certain shields: at places in the combo, with certain weps you may see different damage numbers with a shield strike.
The greatest armors in the game give you 75% Damage Reduction. These are Heavies. If you drop to the best Medium, you get about 57% Damage Reduction. You must roll. I won’t even mention Lights.
AP of 40 would deduct 40% of your total Damage Reduction. So if you’re running at 57%, with every strike I can strip you back to 34% Damage Reduction.
1 Sunder removes 20% of your Damage Reduction, up to a total of 5, for 100% armor stripping.