+P and +P+ isnt a scam to make more money... Nor is +P+ or +P ammo making up for some shortfall, as TASF impiled. First, lets define them:
+P ammo has a a SAMMI standard pressure it cannot exceed. This pressure is higher than "standard" , but probably ok to use in modern firearms. When in doubt, dont shoot it, and talk to the manufacturer of the firearm.
+P+ ammo HAS NO SAMMI standard, it is beyond the +P SAMMI pressure limit. It can be 100psi past the limit, or 5000psi past the limit--theres no standard. Most +P+ 9mm ammo tops out around 40-41K psi. Again, when in doubt, dont shoot it.
There is no SAMMI standard for +P or +P+ in either .357 SIG or .40 S&W---Ammo marked as such may be dangerous to fire, use common sense here. Both of these cartridges are really already loaded to their limits in factory loads.
Personally, I've fired the 124gr +P Speer GoldDot and the 127gr +P+ Ranger T loads in both the 3" XD and the 4" XD and have had no problems in many rounds. You WILL cause more wear and tear on the pistol, mostly to the recoil spring, but other parts will wear faster also. If you e-mail or call Springfield Armory, they will likely tell you what they told me:" +P is ok, but we dont reccomend +P+"(obviously, because there is no standard.)
I WOULD NOT FIRE the +P+ L7A1 submachine gun ammo thats on the market in any handgun-- its well over 40k psi. I have heard of some Glock guys getting 1400fps out of a G17 with this stuff, but I think its playing with fire, even in a Glock.
What do you gain with +P or +P+ ammo? Nothing is gauranteed except higher pressure. Generally, you gain velocity, but there is some ammo out there thats marked +P that isnt high velocity, like the IMI 158gr subsonic "carbine" load, intended to supressed UZI SMGs. So how much velocity might you gain? About 75-100fps for +P and most +P+ loads give 100-150fps more than a standard load with the same weight of bullet.
What does that give you? Depends. In some cases, you'll gain some penetration, in others the bullet is pushed too fast and fragments early, giving shallow penetration. Sometimes that extra velocity also overexpands the bullet, giving less then optimim wound cavity diameter. In other cases, more velocity gives better expansion, and a better wound profile. IF you want to know about a specific load, do some research, start by Googling: FBI Ballistics Tests.
Velocity dosnt equal "stopping power" and +P+ or +P loads may be worse than standard pressure loads in effecting a wound in a target you are trying to stop. "Stopping Power" is a myth. Fact is, handguns are poor fight stoppers, but they are also mor easily carried than a rifle, not to mention the concealment problem. Holes bleeding or central neverous system hits are the only fight stoppers--the former may take a long time if you place your shots poorly, the latter "usually" means DRT--Dead Right There.
Tin cans or badguys------> SHOOT 'EM TO THE GROUND, then asses the situation.