I have been playing with storm box since expansion dropped. I just love the skill! Been trying to build around it and I think this might be best I can come up with.
It has a few Devo and Skill points left open that I don’t know what to do with. Would love some of the Grim Dawn pro’s to take a look
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.
Storm Box might not be a good choice as main DPS skill, though it is a great support spell.
The main issue in your build is you don’t have another damage dealer in your spell rotation.
As above post mentioned, Stun Jack is a great DPS dealer as it can shotgun along with wide AoE spread.
I use Storm Box as support and is great for the followings:
Effective to proc devotions with continuous ticks
Fire and forget as it follows your target
Great debuff with -DA and slow (-DA is stackable with flashbang’s DA reduction)