I know of NO handgun manual (maybe someone can show one) that says +P+ is ok to shoot in it. The reason being rather simple, it will cause more wear on the gun. You need to talk to the Gun Mfgr. directly and ask them if the gun can handle it.
I have discussed this with several manufacturers. Some say, "we don't recommend it" .... and I've had to re-ask them... is the "gun built to handle it" . Eventually they would say "yes" , but we don't recommend it because a "steady diet " (key words there) will cause additional wear on the gun. If questioned further, a "steady diet" to them was anything over 2000 - 3000 rounds of it.
Others , will say NO .. period.
Others will say , "yes" ... it's fine, it will cause more wear over numerous rounds and there are some springs that you may have to replace sooner.
It's typically not recommended in shorter barrel guns, even if they could handle it in terms of pressure, because it cycles it too quickly and may have FTF's, etc. as a result.
As mentioned, there is no "standard" for +p+ ammo, but 10% over +P is what I found some manufacturers state in terms of their ammo.
I have shot both. For my .02 cents, stick with +P at the most, as you won't see any real difference nor advantage in the +P+.