I asked the same question. So here are a few things I figured out.
I have a FNP45 and a FNX9 and have been happy with them both, though neither was threaded. A couple of weeks ago I went about fixing that with barrels from Midwest.
Fitted a FNX45 barrel for the 45 and got a FNS9L barrel for the FNX and threaded it.
Next was to mill the slide for an optic. I dont have any pistols with Optics so I decided to do it for this one. Did not bother on the 45.
The FNX9 slide is just too narrow on top for traditional RMR screw spacing. I was adding a Holosun 509, and it comes with its own plate. I milled it rather shallow, so no cowitness, and added new holes to the mounting plate to fit the narrower spacing on the pistol since the standard spacing is basically centered on the edge of the slide on the top.
Cant go too much deeper or you get into the firing pin channel, and on the right side of the slide there is a large void underneath. My mounting screw actually went all the way through the slide on the right side due to that void. I also had to shorten the OEM rear sight to fit the optic.
All in all though, I like it. Put a few mags through it over the weekend and it is not reliable with the can yet. I think it still needs break in, I hadnt shot the gun that much before all this work.