Dec 27th 2008 9:22PM You're not looking at the whole picture, BRK. Armor isn't the end result; damage mitigated is.
Try to calculate how much dps is getting through on that 13k armor boss, then how much your dps increases on a 11,700 armor boss.
Then calculate how much dps is getting through on a 6k boss, and what the increase is between that and a 5,400 armor boss.
I think you'll find the percentage increase of dps in the two scenarios to be shockingly similar.
Note, I'm not saying armor pen is a great stat, I'm just saying it doesn't scale the way you seem to think it does.