That data will be good for that one gun only. A sample size of one cannot be extrapolated to the whole population, i.e., all Shields. Think of the SD! Nor can data from other M&P models be extrapolated to the Shield. It's a unique gun. A significant sample size needs to be addressed. A further complication of +P+ ammo is the fact that each brand varies since there is no standard for +P+. So, even if one type were to be declared safe in a particular gun, this finding couldn't be applied to any other types of +P+.
I recommend that anyone concerned stick with the recommendations of S&W, namely that +P is OK but might accelerate wear or require more servicing. And, that +P+ is simply NOT to be used, not even a little bit. See page 11 of the Shield owner's manual.