I am not sure how the Shield frame is built. But, with the SD series gun frame, a SS guide rod beats the frame to HADES and back. A lot of SD owners were having problems after installing a SS guide rod.
BTW, S&W will send you a FREE guide rod if you own an SD series gun. I'm not sure if they do it for the M&P (I don't see why not) but I am going to call them tomorrow and ask.