i didnt have time to read the studies that were posted, but there are two really key things i havent heard mentioned in the little discussion about crossbows (and even regular bows) and their ability to punch through armor -- range and arrow type. a crossbow fired from 20 yards that squarely hits will most definitely be able to pierce through a breastplate and multiple layers of mail underneath. it will be much easier as well if it is actually an armor piercing arrow rather than a broadhead as it wastes essentially zero energy pushing the links aside.
on another note, being fired in an arc or nearly straight (crossbows still needed to be arced at least a little, laws of physics (gravity) and such) isnt as important as the overall distance the arrow/bolt is travelling, which admittedly will usually be greater by some amount for the arced arrow. this is because all of the energy lost from the arrow in its flight up will be restored by the pull of gravity on its way back down - unless it hits a bird in mid air
-so the lost energy from the projectiles would be due to air resistance.
a little cool thing i was taught: archers/bowmen, when firing at plate armored targets, would put dabs of bees or osme other kind of wax on the arrow tip to help it stick to the armor and not be deflected -- because thats where the real strength of plate is, deflecting attacks
i dont know if the waxed tips actually helped, but its something they tried!!
btw, first time poster on any forum other than Mount & Blade. realy enjoying the game, and even adventurer mode! just wish i wouldnt keep hitting roadblocks or dying so horribly.....