TeamAFX,
It's not the trigger return spring. The trigger pull on the M&P series is a combination of factors; how far the striker is moved rearward, the firing pin stop plunger spring, the angle of approach between the firing pin plunger and the trigger bar, and of course the striker spring itself. There is also some geometry involved. These are taken into account using the method detailed by Dan Burwell.
There is no benefit in changing just the trigger return spring.