Would suggest bumping it up to +10 damage, equalizing it with Impact.
Piercing spells capstone increases up to +2 damage per cast, being as good or worse than Impact's.
Would suggest bumping it up to +3 damage, making it more comparable to Impact (and equal to full-stacked Cold).
+1 for Slashing, +2 for Piercing, +3 for both
+0 if you wanna make me sad
