I firmly believe that a hand-spanned crossbow launched copper bolt can not defeat an iron breastplate-maille(x2) armor combination and then still has enough power and penetration for a mortal injury. Windlass or cranequin crossbows with steel bows maybe, but they would take a lot longer to reload than the DF crossbows.
I've actually done a lot of research into this over the past year (entirely because of DF). There are all kinds of data out there that proves your hypothesis unlikely.
In terms of anecdotal evidence (purely to add flavor to this discussion), there's a video of a Lego crossbow penetrating 5 inches into ballistics gel from four feet, and 2 inches from 7 feet away. I know that doesn't seem like a lot, but we're talking a plastic crossbow, runner bands, and a plastic bolt.
A pistol crossbow (the one in question) ranged between 50 and 200 lbs pull weight. Longbows at the time were between 80 and 130 lbs.
This study (
http://www.currentmiddleages.org/artsci/docs/Champ_Bane_Archery-Testing.pdf) was done against all kinds of armor, and with "110 lbs pull longbow at 250 yards." Many things were found out in the study:
1. Full plate was less effective against arrows than a coat of plates. It wasn't discussed in the study, but I assume that had to do with the suit's ability to "give" a little.
2. Jack coats were amazing at slowing arrows.
3. Even at 250 yards and with 110 lbs of draw, some arrows could pierce plate. Steel plate.
I know this is a jump from this study to assume what a crossbow can do. However, most people that have looked into the difference between bows and crossbows can tell you that a crossbow wins in strength, while a longbow will win in speed. That said, a pistol crossbow runs at only a fraction of the cock speed of a much large crossbow, has a much lighter pull weight (some heavy crossbows have 600+ lbs), yet still has the same kill range and penetration as a longbow.
If anything, what this means is that there should be more types of bows and crossbows in the game. As it stands, crossbows aren't nearly as strong as they should be if we assume them to be heavy crossbows, OR they're not reloading nearly fast enough if we assume them to be pistol crossbows.