Although, if you can get your DM to rule that Zen Archer Monk can use Wis-To-Hit with Firearms, then you could get away with as low as 13 Dex, taking all the High Dex Req feats with the Monk's Bonus feats.
Also, Cieyrin, you may want to mention (probably in the Stats section) that a Gunslinger with 7 (or less) Wis still has the same size Grit pool as a 'Slinger with 13 Dex.
And that a 'Slinger with low Wis and the Extra Grit Feat has as much Grit as a 'Slinger with 16 Dex. So a Low(er)-Wis build is an option.
(replace Cha for Wis for Mysterious Stranger of course)
Arcane Builder discovery to speed things up.
As written it's unclear whether it takes 75% or 80% of the normal time.