Yea, I head that newer M&P's are apparently comming with Pro Series sear, but since you are looking for 5" Pro Series I don't think this affects you. Aftrerall, Pro Series M&P always came with Pro Series sear.
Is your old striker black and the new one silver? If so, I believe you need new striker spring keeper and striker guide. If you're replacing silver one with silver one and/or are replacing the whole assembly, please disregard this.
Only other suggestion I can think is to look into the striker channel to see if there is anything obstructing. I once found a cotton from the q-tip the owner used to clean the striker channel which was causing misfire.
I believe there is no external modification, it should be legal in both SSP and Production. My last DCAEK install ended up being aroung 6.5 pounds. But if you'd prefer DIY approach, you can try this: http://www.burwellgu...Ptriggerjob.pdf
If you installed "Duty and Carry Kit Springs" only in your 9 Pro, the trigger pull will be heavier. Although striker block spring is lighter than factory striker block spring, both the trigger and sear springs are heavier.