Hopefully this doesn't come across as dismissive or snarky, but since you're asking for opinions, and without knowing user and use case specifics - I'd genuinely recommend looking at a Macro frame/grip module instead of an XL - if you plan on shooting it as much as carrying it.
I regularly carried and shot an XL for a couple of years, but the Macro is just so much more enjoyable to shoot and manipulate than the smaller XL. The XL is not bad at all, but I think the Macro brings a lot more to the table for just slightly more weight and size. I think they both conceal about the same, but if Safariland made a worthy holster for it, I'd have no qualms carrying the Macro in uniform as a duty gun. I certainly can't say that for the smaller module.
I recently sold the XL grip modules I owned and all the XL mags and holsters and I'm all in on the Macro now. If I need something smaller I have other more preferable (to me) options.
Either way, I guess for starters, I'd recommend an extended slide stop/catch/release - that one piece takes a little hassle out of general manipulation for me and streamlines things. If you do go the XL route and you're willing to pay for it, the Boresite Razorback mod is pretty awesome. Otherwise, I'd steer clear of the cheaper Sig red dots marketed/produced for the 365 series and spend a bit more on an optic. OEM Sig parts are great for everything else as you piece it together though.