I have researched the M&P and the Pro versions online only.
Seems that premature wear of finish is a fact and that most if not all target shooters choose to upgrade the trigger with Apex parts. This seems like an acceptable given by those who enjoy shooting it.
So how much does one have to invest in an off the shelf target gun to make it acceptable, even if no real task with acquiring and installing replacement parts?
I am going to make an admission here. Had I left all the parts stock in my PRO my scores would have been no worse nor my performance be any better. In short as far as enhancing the results none of the parts had a material affect on how the gun shot or performed. The PRO as it comes shoots better and is more capable of performing substantially better than I can wring out of it. Proof might be found in the fact my scores using the FS with the PRO parts is no better or worse than my scores with all the Apex parts installed. So to answer your question directly ..nothing.
Just bear in mind the M&P is made for the LEO, Military, Armour Guard market they are not designed as a target gun and the PRO version is not a target gun. The gun has some civilian applications. In Canada they are really limited to Collecting, Range Plinking and playing some of the handgun games where reliability is paramount.
We all talk about the fact one gun has a shorter reset over another but most shooters, me included, derive little to no benefit from a shorter reset because of our modest, truth be known, ability to shoot a handgun with less than stellar reflexs. Ego, pride in ownership, aesthetics, following the mob, are far more important drivers when discussing any particular aspect of parts replacement than a guns mechanical function.
About the only thing that really has enhanced my performance with my handguns are decent competition sights. The PRO comes with those. Now some sports do require longer shots on small targets and lighter trigger pull can certainly help but so would decent instruction and practice. The stock PROs trigger pull is around five pounds which is quite adequate for what I do with a gun. The 3 pound pull I have obtained via the Apex parts is 2 lbs lighter. My scores did not improve 40%.
Top end shooters will get small advantages in part replacements that will differentiate them from other top shooters but, from the matches I have shot, most shooters who perform less than the top shooter would get more out of their money by paying for quality instruction from quality instructors for the discipline they want to participate in, than replacing a host of parts. Some might not improve at all from the instruction but they can say they attended xyz class.
I an however, a parts whore. If an ad says a new sight will turn me into another Vogel I am all over it. Alas about the only thing I often gain is a small amount of buyers remorse. John R still out shoots me by minutes and Troy H by seconds.
If you must replace parts and you are talking the FS my suggestion would be to get the Apex striker block and a lighter trigger return spring. Neither is very expensive. The trigger feel will improve. If you are buying the PRO spend the money on ammunition, practice and some instruction.
Premature wear on the finish is not a fact just the experience of some folks. My original M&P has some rub areas on the front edge of the slide from drawing thousands of times out of a kydex holster but that is to be expected. The slide is made of stainless steel. Not sure what the issue would be. The frame is polymer.
Take Care
Bob
ps A new shooter who works for the Sheriffs department shot his first IDPA Classifier last week. First time out using his duty gear (Triple retention holster and duty mag holders) and duty pistol shot Sharpshooter and almost made Expert. His pistol was an off the shelf 9MM M&P FS with a stock trigger. He served three tours in Afghanistan before leaving the Army. The gun or the shooter you decide.