The main problems in adding a feat to any items is two-fold.
1) You grant access to it to anyone with UMD. (Which is very nearly every pistoleers around, so the class restriction is moot anyway).
2) Any free feats on an item is a feat a player no longer has to invest in at level ups. This takes away from the necessity of making character building choices and that should always be avoided. It's even worse when it's a feat that would otherwise require other feats to acquire.
I'm not against a feat on an item if it is to address a game balance issue, or something so minor no one ever takes that feat anyway. But "Gearling's Superposed Loading Technique"? No way. You would not only give a free feat. You'd also give one that normally requires an investment of 2 more feats to acquire. It would be most unfair to players actually taking the time to invest in it to acquire it. Even Delven's Maneuver is too much.
It is not a problem that PBAs would acquire the similar feat at a later time. A PBA is still free to invest feats beforehand to get it sooner, or can be patient investing in something else knowing that his sacrifice will pay at a later time.
1) Well, no the class restriction is still there regardless. People do have to invest into UMD to get it if they want to get it. That means popular builds like Gun Monk aren't going to get it. It means full fighters aren't gonna be using it. That leaves bards, rogues, beguilers and assassins to use it, or multiclasses of those.
2) Why is Delven's Maneuver too much? Nobody is ever going to take only one of the automatic reloading feats. You still will have to take Delven's Maneuver on nearly every build because Gearling's is too necessary for pistol builds. Items do not grant prerequisites for feats. And believe me when I say, a 30% of backfiring is not gonna be efficient to deal with when it gets to the short and curlies of mechanical play.
In addition, I just simply do not think that's how builds in PotM work. I wholeheartedly think that it actually is quite a problem. If you have redundant feats in your build, you aren't building optimally. I wouldn't recommend to someone that they take Skill Focus: Parry and Improved Parry, for instance, so they can hit their parry cap at 14 instead of 17; if I suggested that advice to someone I'd get some looks. Why would someone who plays BPA need to have build redundancy for something that is quality of life?
We've adjusted classes in the past with the intention of quality of life, and we've also accepted items in the past that have mildly made some feats redundant, such as the Hexblade ring of Arcane Stillness. It gives automatic still spell 1, which allows arcane casting of spell level 1-3 while in heavy armor. The item has drawbacks compared to taking Battle Caster as a feat. However, it creates a quality of life "tie over" for some people who would like to wear heavy armor and cast spells, similar to what I'm trying to accomplish with giving Delven's Maneuver as a feat. If a BPA or UMD invested class uses the gun, it has significant drawback and also stands as the only firearm that benefits from the feat and it does not facilitate the automatic reloading of gunpowder. You still have to take Delven's Maneuver and the other prerequisite for Gearling's because, if you are doing a build that focuses on guns, you HAVE to get Gearling's Superimposed Loading Technique or some other method of automatic reloading. For a Beguiler/Bard/Assassin who UMDs and uses the rifle and doesn't have gun feats invested, it is little more than a gimmick.