I don't see how +7 pierce equates to +3 pierce (even with +56% damage). Maybe i'm being dense but I don't see it.
If its an attempt to balance the lack of bonus from your build then why apply it in the loomed version and not the normal unloomed version? Spent more loom points? So what, why should they combine for more damage just because you spent money on it?
Why do you even expect a bonus damage when you don't even have to spend points towards using the thing? Bows spend 4-6 points on PD and WPF is much more important. Melee spends points into shields, PS, IF etc just for their melee build. Xbows shouldn't get any bonus at all at any rate without spending points into something