I know others here have P365's of various types. I don't recall specific upgrade discussions except for the Romeo Zero vs. Holosun one. It's possible I missed it, I'm a more sporadic poster/reader, a short search didn't seem to show anything. I'm still using my Romeo Zero. The funds for the Holosun keep getting redirected into other things including modernizing my M16 for subgun competition. In any event I thought I'd let you all know my thoughts on some other upgrades I've done. Maybe it will help someone else, maybe not.
Early on I did the MCarbo springs upgrade. I forget if I may have mentioned that here in thread before or somewhere else. It definitely helped the trigger a lot. There was still quite a bit of takeup, but the break was crisper and a bit lighter. I'd say just about 5lbs or so based on my trigger pull gauge.
I also added manual safety to mine. That of course is a whole can of worms, but with a lighter trigger and IWB carry personally I feel safer with the manual safety. I don't have the body type for Appendix carry, but I would think the same thing for that. Growing up and learning to shoot with manual safeties it is pretty natural for me to thumb the safety on the draw. I know that mileage varies for many, and many absolutely hate manual safeties so FWIW, just my opinion here.
Awhile back I upgraded the MIM striker to the MCarbo titanium striker, personally I could not feel any difference. Supposedly it can reduce lockup time. For me it was more of a reliability thing. I never had an issue with the factory striker, but there are reports of them breaking.
Most recently I did two additional upgrades. First was the MCarbo flat trigger. This trigger also changes the profile of the engagement between the trigger and sear bar I think it is called. This to me was a fantastic upgrade. It reduced the take up/travel to almost nothing, and according to my trigger pull gauge gave me another about 1/2 pound reduction in trigger pull weight. I'm consistently getting just over 4.5 lbs now. I know there are other options out there for a similar trigger upgrade. I went with the MCarbo based on having good luck with their other products. I think the point is the factory trigger leaves a bit to be desired.
The other most recent upgrade was a Wilson Combat grip frame. I've only function tested it so far, but from the feel of it, I do think it is better at least in my opinion. It gives a slight grip angle change that I like more. The grip is quite a bit more aggressive which can help tame a short barrel 9mm. I will need to see how it does shooting a match, if it becomes uncomfortable, possible abrasive to my hand firing 150 to 250 rounds in a few hours, vs. the couple magazines i did for function test.
In any event hopefully this helps someone else thinking about upgrades to their P365's.