My suggestion would be to shift some of that bonus damage to its baseline so we see something along these lines:
Stormbolt: Hurl your weapon at an enemy, causing 250% weapon Damage and stunning them for 3 sec. Deals an additional 250% weapon damage to targets permanently immune to stuns.
That way in pvp you are able to get a decent size hit and sames goes to mobs in pve and it still stuns, and you still will do the same amount of damage to bosses.
Tl:Dr Stormbolt should have some of the bonus damage shifted into its baseline damage to make it worth picking when facing Pvp players and non boss npcs.