The +P and +P+ designations and the NATO standard are set by different organizations, for different reasons.
The U.S. ammo makers, up until the "wondernines" came on the scene, did little development for the 9m.m. x 19 round. U.S. ammo was widely known to be underloaded or loaded below the 35, 000 psi level. U.S. 9m.m. ammo would in many cases be in the under 1,100 fps for the 124 and sometimes the 115 grain bullets.
Things have changed a lot.
The U.S. military wants to hot load their ammo. A recent article in a trade journal stated that the U.S. army wanted to raise the pressure maximum of their ammo to the same level that they use in the 120 m.m. cannon on the M1 Abrams tank! That is 70,000 psi or at least as near as they can get.
This way, they can get higher performance without changing guns, which is expensive.
You have to remember, that many of the people coming up with these ideas are not looking at all this from a civilian use perspective. It may be possible to raise the pressure level for an M-4 carbine, but the short barrel which caused a performance drop from the M-16A2, may make this impossible without a larger case, like the .224 Valkyrie or .22 Nosler.
Whether they will or just go to a larger caliber, is still up in the air. As long as they are being promised more money by the President and Congress gives it to them, the idea will remain alive as a project which may or may not produce anything, but will help improve the promotional potential of the people involved.
The 9m.m. ammo specifications are probably more European than American. The U.S. military buys pistols, not because it wants to, but because M.P.'s and security forces need them. So do Special Warfare units and others where space is an issue. Pilots, tankers and non-infantrymen, like mortarmen also have a need.
So the European specs, which have always been hotter than civilian U.S. ammo makers have been more important to the military, which really does not care about handguns.
If you disagree, fine.
Now, with the proliferation of body armor, velocity and penetration have become more important. In an insurgent war, a pistol should have maximum stopping power as you are likely to be in close contact with an un-armored opponent who is likely to be the one that initiates combat with ambushes. You are not going to see jihadist battalions marching up the road to attack U.S. bases when a radio call will bring F-15E's, F-16's, F-18's or worse, Apache gunships or A-10 attack aircraft. At the very least, you can expect rapid artillery attacks.
In a near peer war, as the new buzzword calls it, you are more likely to be fighting an organized, heavily armed and very likely body armored opponent, so now penetration is the desired effect. When the NATO round was poor in stopping power, it is good in penetration and a steel tipped or tungsten steel tipped round would be very good.
It all depends on your situation and needs.
When I started in law enforcement, we had the option to carry a 9m.m. that we had to purchase from an approved list. I bought a GLOCK 19 and then a 17 and also a SIG 226 to find the one that fit me best. We had to use the standard issue ammo which was 115 grain WINCHESTER jhp +P+ for carry and qualification with standard 115 grain for practice. We later went to the 124 grain FEDERAL Hydra Shok +P+ for carry and used standard pressure Hydra Shok for practice! The velocity on the Hydra Shok was not much higher than the standard pressure load which was not that fast either. The noise, flash and muzzle blast were very noticeable with both +P+ loads and when I recently did my qualifications using my personally owned guns, the +P+ ammo was still loud, bright and less pleasant than the standard pressure ammo.
I recently bought a BERETTA M9A3 and wanted to try it with the NATO load and +P and +P+ ammo. You immediately notice the difference compared to when shooting standard pressure ammo.
The NATO load is a +P+ load as far as I am concerned, based on the noise and muzzle blast. It may be borderline with +P for pressure, but feel's like touching off a 110 grain .357 magnum. The +P ammo is almost as bad when fired from a shorter barreled gun. I have used a 3 inch and 3.6 barreled pistols and think it is borderline for the improved performance versus the less pleasant shooting.
For me, I will stick with standard pressure FEDERAL HST ammo in my 9m.m. pistols.
I know that a number of law enforcement agencies went to the +P+ ammo to get some real performance out of the 9m.m. The Illinois State Police found the FEDERAL 115 grain jhp +P+ ammo did exactly what they wanted and my agency had no complaints about the performance of our 9m.m. pistols with the +P+ ammo. I think that the premium bullet ammo that all the ammo makers are not providing perform well enough that you can skip the +P+ and even the +P ammo in the 9m.m. if the barrel is at least 3.5 inches.
However, one advantage of so many choices is that you can suit your needs and suit yourself.
Just my 2 cents worth
Jim