Here's a link (with more links) that you may find interesting. It mostly concerns one particular brand of pistol, but the basic ammo-info can be applied to them all.
http://www.hipowersandhandguns.com/9mm_hi_power_ammo_selection.htm
My own opinion on the 9MM is this;
My #1 concern with any ammo in any pistol is "will it feed reliably"?
Even "regular" pressure (non+P) 9MM ammo can be had that ventures into the same ballisic territory as the +P 38SPL.
That aint shabby.
A +P 9MM is close to the old 38-Super, and the +P+ version(s) are right there with it. What that tells me is that a steady diet of either is probably beyound what the design paremiters that most "stock" 9MM pistols are made to handle. That, and anytime we are using a smaller case while getting equal velocity from X-amount of bullet, we are also dealing with more pressure than we'd see when using the larger case. That may be something to bear in mind if you're using a gun that has a chamber design with less than optimal case-support, or if you want to get a lot of use out of a stock pistol before undue wear sets in.
I aint sayin' that using +P (or +P+) ammo will blow up any certain gun. I'm just sayin' that it'll certainly batter it more than "regular" ammo.
To me, the questions of stopping power is....when is enough, enough?...and at what point do we abandon trying to make a .36 perform like a .45?
After all, dern-near any pistol that is available in 9MM is also available in a .40 or .45, eh?
Hope this helps.
DGW