As far as I have read, the only gun with the improved trigger is the Shield. It has a different design than all the other M&P's. It has a positive reset "click" sound and you can feel the reset as the trigger jumps about 1/32 inch at reset. It has been rumored that in the future S&W will adopt the same trigger design to all M&P's, but they have yet to make that change. I do know the Shield trigger bar is more narrow than the standard M&P trigger bars and will not fit any other M&P models.
Bob