As far as i know, nothing increases damage, its just what crossbow and bolts give.
That is reason why crossbows are not as strong compared to bows/throwing as they seem with quick look.
About your build, dont see any major flaws in it, be ready to shoot quite small ammount of bolts in quite many rounds, caused by your long reload time. Also it is possible you end up being more accurate with heavy crossbow (hardcoded inaccuracy from damage).
I myself used to play with sniper crossbow before new exp/gold system, these days it has been light crossbow (and most of time horse).
i dont specialize on builds and analyzing them, but cant see any major flaws in that one, as long as you do understand how much of time you are going to spend reloading instead of looking at your bolts flying.