It's plain and simple. If the manufacturer of your weapon recommends you DON'T USE +P ammo, then DON'T USE IT. You'll void any kind of warranty and accelerate the wear and tear on your weapon. And, most importantly, it is dangerous to you and people around you.
There is a plethora of fantastic standard pressure SD ammo out there. Using +P in a non +P rated weapon is foolish and gains you nothing.