I've done a lot of trigger work on M&P's, including my own. If you look at the sear, they have a sweeping configuration that causes the striker to move slightly backward, to finish the cocking, when you pull the trigger. If you do a Burwell trigger job, they remove some of this backward sweep, which reduces the trigger pull from 6.5 to around 4lbs.
If you completly remove this backward sweep, it will become a true single action and an even lower trigger pull.