I occasionally carry 9mm Hi-power.  When I first started carrying a Mk II about 40 years ago I was always told to use only non +P ammo.  Later when I got a MK III I continued to use non +P ammo.  While I don’t shoot these pistols often I do shoot about 500 rounds of my carry ammo through them.  
I was told recently that I can use +p ammo through these pistols while others told me I can use +p only in the MK III.
So what’s the real deal on use of +p ammo use in Hi-Power pistols?

A steady diet of +P is going add extra wear on pretty much any pistol. If you shoot a lot of it (more than a magazine every once in a while) get a stiffer recoil spring Vs the standard OEM spring.

check bhspringsolutions  for different recoil springs depending on what you're shooting a lot of.

Gunsmiths used to alter the more stoutly built .40 HP’s into +P 9mm capable HP’s.
The standard HP slide is not rated for a regular diet of the hot stuff.