Beware that a hard metal magazine follower on 870s, 1100s, and 11-87s may beat out the retaining lip at the receiver. Had that happen with my original '63 model 1100 and the carrier started hanging on the original steel follower. I like the aluminum ones from Brownells.
The original trigger guards are cast aluminum, not pot metal. I actually like the synthetic - space age engineered polymer on an Italian gun, plastic here - ones better except for the molded in loco because the edges don't get silver. I have yet to see one with a problem.
The 870 P gets extra vibrahoning of the internals, the sear spring is stronger, the carrier dog spring is stronger, and the magazine spring is stronger. Everything else is the same.