One of the guntubers I mentioned above put the 17 round mags in an 365xl, and they worked fine. Interestingly the new 17rd mags are shorter then my current 15rd mags.
They also successfully swapped slides between the two, so that leads me to believe that you could just get a macro grip module and run it on your 365xl innards as long as you don't mind skipping the comp. If you were going to get a short barrel, comped slide, and new grip module, I think you'd be close to the price of the new gun.
Also, in case anyone wondered, T-rex arms sent me an email saying they have a holster ready to go for the macro, but that it does NOT work in p365xl holsters, so take that for what it's worth.
I find the concept interesting, but I'd like to shoot one before I commit to over a $1500 or so of new carry pistol, holster, and optic.