Changing mode between extra damage vs shields and extra damage vs armor is OVERPOWERED.
Balance > realism
No, if you change the damage values of piercing weapons (which has sense, with the huge amount of MINIMUM damage they deal). non loomed Steel pick around 28 piercing damage, and the others start going down 27, 25, 23 etc.
The broad axe could have an alternate mode doing around 22-24 piercing damage, it would be ok.
Think of it, piercing weapons are very good bypassing armors, but they rarely inflicted heavy wounds due to the limitation of the length of the tip. Also we could make them unbalanced, cause when they hit (also on shields), they usually got stuck.
Anyway, the way and VALUES OF DAMAGE of piercing 1h weapons right now is a bit overpowered.