Path firearms are not weak as long as they're at least commonplace enough that you don't need to dump feats to get them and you can use gunslinger's dex bonus to damage, or do some other shenanigans like the adding the "guided" quality with a WIS based build.
Check this out: firearms hit touch ac instead of normal ac within first range increment. https://docs.google.com/spreadsheets/d/1E2-s8weiulPoBQjdI05LBzOUToyoZIdSsLKxHAvf8F8/edit#gid=3
Look at this difference between normal AC progression for enemies vs. touch. Generally most enemies have sub-20 touch AC and bosses sub-30 up until very very late stages of the game. This allows you to easily combine deadly aim and two-weapon fighting + rapid shot without sacrificing your ability to hit consistently with shots later in the combo. Once BAB hits 11 (Gunslingers have full BAB), you have 3 attacks with main hand pistol + 3 attacks with off-hand pistol + 2 more with each hand while rapid shooting for 8 attacks that will still reliably hit most enemies, even if you deadly aim for -3/+6.
Monks and two-weapon users often have extra attacks but at a +3 for the 6th attack are never going to hit with them unless the enemies are severely debuffed. Two-weapon gunslingers can hit almost always. At CR 20 the median touch AC is 13 and regular AC is 37. A gunslinger's 6th attack needs to have +8 to hit pretty consistently, while a monk needs +32 (or +18 to even have a chance to hit outside of a critical).
Deeds can be pretty good but a lot of them are useless. And unlike ki you get to replenish your pool pretty frequently, so whenever you secure a kill you get back the grit, meaning you get this once per enemy for free. Plus another grit for each crit along the way.
So for my onion here, being able to unload your grit for the day into an enemy for a couple of d6's per grit point, and then get at least one of those refunded from the kill, is pretty sweet as is. I don't think this is a deed to make a build around, but it's one of the better ones just for letting you add a substantial amount of damage.
Adding half-scaled rogue sneak attack damage with zero conditions to prop for 1 grit * per turn* seems pretty busted, especially because that grit would get refunded every time you got a kill (which you would be getting every turn).