Boss base armor is 10,643. Sunder and FFF are multiplicative, and together are 24%. They reduce boss armor to 8088.
Then ArP comes in. And some funkiness.
Armor penetration stacks with itself - the more you have, the better it is.
Armor penetration rating now caps at 100%, or 1231 rating. Because it stacks multiplicatively with sunder and FF, and because of the cap on the amount of armor that ArP can affect, the lowest you can possibly get a boss's armor now is 315, or 2% mitigation.
Hmm, I think I see what I did wrong. I guess I was counting ArP additively with the debuffs, instead of multiplicative.