Alternatively, adding a skill requirement like PD for xbows would be acceptable, if it allowed xbows to not suck any more.
No thanks. If I have to choose between putting points into a skill for crossbows or putting points in for archery I'd go with archery every time. Unless of course the skill gives a significant shot speed and damage boost. Of course the whole point of crossbows though is that it doesn't take much training to be decent with them but a dedicated archer is better in most cases.
As far as the current damage goes. With 135 wpf and a masterwork siege crossbow I can down some stationary people with a chest shot. Those that survive are often severely wounded.