@AK103K
Personally, I wouldn't feel comfortable shooting +P+ loads out of a gun if its owners manual advises against it, unless it was a military/LE grade firearm which is obviously suited for +P+ since that's what a lot of LE uses and military uses hot NATO spec ammo.
Yeah, it would most likely be fine shooting just enough to ensure that it will cycle reliably under ordinary operating conditions, but I'd rather just get a slightly larger pistol like a Glock 27 or an M&P40 Shield then convert it to .357 SIG than shoot overpressure ammo out of a gun that wasn't designed for it like a Ruger LC9s or SIG P938.
Ah, but then again, I'm really not much of a +P guy. If a standard pressure loading doesn't do it for me, then the 10-15% increase gained by +P ammo won't either, so I just go straight to a more powerful cartridge. Sure, it's obviously cheaper to just buy hotter rounds than it is to buy a whole new gun, but +P and especially +P+ loads cost more than regular ammo, and if the stock recoil spring is too light to cycle +P+ reliably then you have to buy a heavier aftermarket recoil spring to fix that, and honestly if you feel that what you have is insufficient, then why not just trade it in for something more powerful anyway? But again, that's just me, and I'm not one to go telling others what they ought to be using for self-defense.