Here's my 2¢ on the subject, and it comes with my personal guarantee that it's worth every penny that you paid for it...
.45 ACP is a cartridge with an almost legendary reputation as a manstopper dating all the way back to the early 1900s. The .45 ACP was designed to replicate the performance of the US Army's "M1882 Government .45 Caliber Ball" load which was essentially a .45 Schofield load with the ballastics of a light .45 Long Colt, which had served the cavalry well for decades and had a reputation for being capable of readily stopping a galloping horse dead in its tracks. Keep in mind that during WWI, horses were still widely used on the battlefield, so a round that could stop a horse was a desirable trait in a military service cartridge.
Not to be outdone by it's predecessor, the .45 ACP cartridge went on to earn its own reputation as a reliable man-stopper and faithfully served the US Army from 1910 to 1985, and is still in use within certain branches of the USMC/SOCOM. Yes, it has been largely phased out in favor of 9mm NATO, but that change, regardless of what people say, was largely for the sake of adopting a NATO standard cartridge, which was never a popular decision among soldiers. Also, it's important to note that while 9mm NATO will get the job done, it's an overpressure variant of 9mm Luger with pressures in excess of even SAAMI Spec 9mm Luger +P loads, which brings me to my next point...
+P ammunition is by definition a high pressure load which exceeds standard loads by up to 15% which increases chamber pressures substantially, but in exchange for a marginal increase in performance, so unless you're carrying a heavy duty firearm, usage of +P ammunition doesn't seem worth the trade off of decreasing the service life of the firearm or at least its parts among casual shooters. (And yes, if you're going to carry +P ammo, then you should train with +P ammo as well because your POI will generally shift considerably between Standard Pressure and +P loads.)
So in my honest opinion, if you desire better ballistics performance than what Standard Pressure loads have to offer, then you're better off switching to a more powerful cartridge, that way you'll get the increased performance that you desire with a firearm that won't wear out any faster because it's already designed to fire the cartridge it's chambered in, unlike most firearms which shoot +P ammo. (And no, "Rated for +P" does not mean "Designed to shoot +P" it merely means that it can safely shoot +P, and Owners Manuals generally reflect this.)
The S&W M&P45 Shield is without a doubt a well-made pistol, but it was designed to be a lightweight carry pistol, not a heavy duty service pistol. Yes, police might use the M&P Shield, but as a BUG. So not only will +P loads accelerate wear on the Shield, it will most likely be more difficult to shoot fast/accurately due to the increased recoil of +P loads out of such a lightweight pistol. Not to mention that out of such a short barrel, it's most likely that the benefits of +P ammo are wasted because powder doesn't have as much time to burn and that extra pressure doesn't have much time to build up behind that bullet before it escapes out the barrel.
So yeah, if you want more power than what Standard Pressure .45 ACP loads have to offer, then you're better off looking into 10mm Auto than using +P ammo, and I'm talking about real full-power 10mm Auto by Cor-Bon, Double-Tap, or Buffalo Bore, not common 10mm Auto Lite/FBI loads which are essentially just .40 S&W in a longer case. But for that, you're going to need a bigger, heaver gun than the Shield, and you're going to have to move away from Smith & Wesson since the only 10mm cartridge they care about is their own .40 S&W cartridge, which really doesn't offer much of anything over .45 ACP.
That all having been said, while the legendary man-stopping reputation of the .45 ACP cartridge may be an exaggeration, as far as handgun cartridges go against two-legged threats, it's certainly among the best there is. After .45 ACP, handgun cartridges start to beget diminishing returns, because while they travel at higher velocities and strike with more energy foot-pounds, none of them but the mightiest of magnum cartridges have the necessary mass, velocity, nor energy to generate remote wounding effects in flesh, so unless you plan on carrying a .50AE Desert Eagle, 460XVR or a M500 Magnum, .45 ACP is just about the best performance you're going to get out of a semiautomatic pistol.
You don't need to use +P ammunition either, Standard Pressure .45 ACP was long considered the gold standard for semiauto pistols, and regardless of what unilateral testing protocols conducted by the FBI/IWBA may suggest, any hunter can tell you that bigger, heavier bullets have much more dramatic effects on bone than smaller, lighter bullets do, and when the objective of self-defense is to stop the threat by incapacitation, obviously shattered bones tend to be more debilitating than cracked or broken bones with holes in them. Hit a deer in a non-vital area like the shoulder or the hip with a smaller, faster bullet and it will keep right on running until it bleeds out, leaving the hunter to track it down and finish it off. Hit a deer in a non-vital area like the shoulder or the hip with a bigger, heavier bullet and the shoulder or hip breaks, the deer goes down and can be humanely finished off on the spot. Now obviously in self-defense your next course of action after dropping a threat is to flee to a safe distance then call 911 immediately so that the police/paramedics can arrive, but you get my point... Just like a large game animal, a threat will go down if a .45 ACP smashes through a bone which is necessary for them to remain upright, whereas a smaller, lighter bullet like a 9mm might merely punch a clean hole through a bone.
