Hmm, the enhance arrow options are weaker due to the lower rate of fire with early firearms, and phase arrow is of limited use, as guns already strike for touch attacks in early range increments. Guns mostly have lousy range as well, relative to bows, which takes away the big perk of Imbue Arrow, which is that you could put an area spell on a bow and launch it well beyond your normal range limits.
Gains:
-Touch attacks in close combat increments.
-Using gunslinger gets you Dex to hit and damage.
Losses:
-Misfire means at high volume of fire you're liable to suddenly lose your primary weapon
-No ability score increase applicable(not without gunslinger)
-Useless underwater
-Low range increments, which limits how far you can chuck your spells with one.
Costs:
-More feats to get firearms to a good rate of fire.
-Feats to deal with misfires(easy generic way is to ditch the broken gun for a new one with Quick Draw, as Enhance Arrow ensures it works anyway. If you're made of cash you can also just keep a whole stack of loaded guns to quickdraw instead of reloading)
-Costly ammunition at early levels, you're talking 11gp per shot here.
Theres a few compelling reasons to use one I guess.
