I’ve got a one year old 365 and a 3 month old XL. The 365 has 2600 rounds through it with one slight feeding bobble shooting weak hand only the first range trip. (I recently did my first recoil spring replacement on that gun.) The XL has 460 rounds through it. I had one light primer strike (at about 300 rounds) with a handload that fired on restrike. The guns are totally different animals in terms of springs. The 365 has a dual recoil spring that is much stronger than the single recoil spring on a guide rod found on the XL. The spring tension appears correct on the 365, but the XL may be slightly undersprung. If I chamber a round from a fully loaded magazine and do a press check with the 365, it will spring back easily. The XL slide springs back slowly with a clean gun to barely, or not at all, with a gun that has been shot a bit. However, even with the XL, it does not appear to matter as long as the slide has enough momentum from the slide cycling. The hotter the ammo, the greater the slide momentum going backwards and forward. I learned a lesson with 9mm Glocks, which are somewhat oversprung, to shoot duty strength ammo right from the box, then work into lower strength ammo after the gun has broken in. That practice appears to have worked with my Sigs, and the only exception has been one of my IDPA handloads in the relatively new XL. (They run fine in the 365.) I have no concerns carrying either gun with self-defense ammo.
A couple of other points. First, some have mentioned on forums that the striker channels on some 365s have lube in them, which should be cleaned out. Second, Sigs generally are tighter than some other guns and they have much longer rails than, say, Glocks. I use more lube on Sigs than I do on Glocks.
I suspect that Sig will do a rolling upgrade with the XL recoil spring. I say this because the slide is optic ready, and the first Sig red dot sights are about to be shipped. Adding any weight to the slide can only complicate cycling with an undersprung gun.
I hope your situation gets resolved satisfactorily. I would be interested in the details of your malfunctions, including ammo used and round counts.