Stormbox scales very poorly into ultimate game. It has every disadvantage a caster skill can have - cooldown, no weapon damage component, low AoE, low natural damage, and it’s a non-stacking DoT (i.e. it gains no damage scaling from cast speed or CDR, two critical caster damage multipliers). My elemental PRM Mage Hunter was planning to use it as one of my trio of damage spells (PRM, Word of Pain, Stormbox), and I ended up cutting it entirely because the damage it adds to my rotation was too low both as a main attack and a support skill to be worth the points.
It’s pretty great early game (though Word of Pain is more awesome IMO), but late game, if you want to still focus on lightning damage/close range casting, you will be better off retooling your build into lightning jacks.