i wouldnt advise leveling up must pierce force
sure, every little helps, but this is just too little
lets say your lv40, your imbue adds 117~195 dmg to your attacks
now the 10% more mag. dmg would result in +12-20 dmg wich is (in my opinion) not worth the sp spent here
get parry instead. its a little overall dmg reduction (wont work wonders either, but i say its more useful for str chars by far)