I found that the P365 12 rd magazine would sometimes pinch my hand and the grip felt squishy. I installed the XL grip module and the P365X feels more solid in my hand. Many people prefer the beavertail on the XL grip module for better control during recoil.
The XL Grip Module magazine well is also slightly flared to aid with magazine insertion.
Top: P365XL Grip Module
Bottom: P365 Grip Module
If I ever feel the need for the shorter P365 grip module, I can swap the grip module in less than 30 seconds.
BEWARE and be careful when you remove the FCU from the grip module for the first time. If the tangs on the sear spring are too short, or if the sear spring tangs were installed incorrectly, parts of the FCU can fall off. If the sear spring tang length is okay and the spring is installed correctly, there will be spring pressure applied to any part that could otherwise fall off, and you should be able to violently shake the FCU and the parts will remain installed.
The P365 12 rd magazine will NOT fit into the P365XL grip module because a difference in the floorplate design. The P365XL 12 rd magazine will fit into the P365 grip module, but there will be a huge gap.
To deal with the magazine incompatibility issues between the P365 12 rd and the P365XL 12 rd magazines, I modified the P365 12rd magazine sleeve. I sanded the bottom of the sleeve adapter and sanded the nose of the adapter so that if I want to use the P365XL 12 rd magazine with the P365 grip module, I only need to slide the adapter onto the P365XL 12 rd magazine and it will insert into the P365 grip module without any gap.
I still need to sand with some smoother sandpaper, as I have only done the rough sanding of the adapter with coarse sandpaper so far.
Also, if you want to use the P365 12 rd magazine with the XL grip module, you will need to replace the floorplate with the XL style floorplate.
Left: P365 12 rd Floorplate............ Right: P365 XL 12 rd Floorplate