Scott Springer observed that the P320s have a left bias so if you have your sights perfectly centered, chances are still that the POI will be slightly left until you adjust.
For me dry fire brought the pull weight down a pound. Basic polishing brought it down another pound. I have one gun that am testing the Grayguns drop-in kit in. That one is at or slightly below 3lb; however, Bruce plans on selling just the trigger if you don't want the whole kit. The new geometry on the trigger itself seems to be the equivalent of going to a Glock 3.5lb connector. In the P320 just dropping that in takes you down another two pounds or so.
The kit also has a new sear, reduced power sear springs, and also modified triggerbar spring. No idea on the price but based on other GGI kits and how much he has invested into R&D my guess is that you will save time, but not a lot of money by doing the full kit yourself.