I think the TWF build is probably the "best" in terms of damage output. Stack as many static bonuses to damage as you can and then just concentrate on making as many attacks as possible.
The Mysterious Stranger stuff doesn't work with Spell Cartridges out of the box, since Mysterious Stranger gets Cha to Dam as a swift action, and Spell Cartridges needs that swift action for Arcane Strike.
The best workaround I've seen is Bloodrager's 'Blooded Arcane Strike' feat - as long as you're bloodraging, Arcane Strike is active.
Also, if you want to go TWF, 1 level of Eldritch Archer Magus gives you the ability to ignore somatic components as long as you're holding a ranged weapon.
So you're looking at 1 level of Gunslinger (Cha to Dam as a swift action, firearms proficiency), 1 level of Eldritch Archer Magus (Cast spells while holding guns, Spellcasting at level 1), at least 3 levels of Trench Fighter (Dex to Dam), and then Bloodrager the rest of the way (Cast spells after level 4, Arcane strike w/o swift actions).
More Fighter levels open up Weapon Specialization and Weapon Training at Fighter 5(including effortless dual wielding).
I'm not sure if a 2nd level of Magus (for a rapid-shot equivalent Spellstrike) is worth it.