I agree the Apex USB is the big bang for your buck when upgrading an M&P.
I've now done Apex installs in multiple M&Ps, and the USB is the biggest jump in performance. The process of swapping the trigger is not difficult, you just need to be careful and follow directions. The first install I did I watched the videos through a couple of times, then watched the portion pertaining to each new step before starting.
The most recent install I simply tore down the M&P and installed all the parts without any need of reference. I was pretty pleased with myself
Rmoving the rear sight on M&Ps is a pain in the ass. Do not mess around with a sight pusher. You will need a vise, punches and a hammer. I typically need to remove factory installed sights with a steel punch, which damages the sight. So I always plan to do a sight upgrade during the process of installing the USB.
Every sight I've installed on a M&P has required significant stock removal to properly fit the dovetails. I've found diamond files to be invaluable for filing down the sight. I use non-marring brass or aluminum punches for installing new sights, once I've filed them to fit.