Back to the manufacturer's liability for a moment.
The ammo manufacturer has a lot at stake if they supply a cartridge that results injury to the shooter.
Winchester / Olin is a reputable company, and it seems unlikely to me that they would sell a 9mm round that would cause a commonly used law enforcement gun like an M&P 9c to fail catastrophically -- at least without all kinds of written warnings.
Perhaps they would, however, sell a round that results in increased wear and tear and higher maintenance if used constantly.
Since the ammo manufacture knows the pressure of their own ammo they are in a position to make a responsible judgement on the relative safety of a given round in a given firearm.
The weapon manufacture has no idea what the pressure of a +P+ is because there is no industry standard. There is no SAAMI standard, there us no U.S military standard, there is no NATO standard.
The fact is +P+ is a made up marketing term that could mean anything. Its like "new and improved" or "rich Corinthian leather".
How can a firearm manufacture say their firearms can handle +P+ when no one knows what it means? They can't.
My point is if you want to know if a non standard round will work in a particular gun, you go to the people that made the ammo not the people that made the gun, cause the guys that made the gun are going to say don't use it just about every time.