Personally, I think less is more. My Ruger SR9c had a magazine safety and I removed it. It's a piece of metal, the shape of a U and extends through the slide. When a magazine is inserted, it pushed the safety up so the firing pin can clear it.

I don't know how they are on M&P pistols, but on the Ruger's, they just barely clear and it was visible on mine that is was dragging on the firing pin when fired. This can lead to light primer strikes. After removing mine in the SR9c I can see a visible difference in the impression of the firing pin on the primer.

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Rod
M&P 22A, 40c, 15A, 15-22