I don't have an FN, I have P320s.
That said, I have the Parker Mountain Machine micro comp and two of their twin port ultra comps. I run the twin port comps now, the micro comp and barrel is not being used.
What I found with these is that my holsters worked fine, the comp may just stick out a little bit.
In other words, if you simply bought a p320 holster for the longer P320s, it would fit as well.
Which, is what I do, one of my P320s is the 3.6" slide, one is the 3.9" slide, again, both have the twin port compensator on them and I buy holsters for the Sig X5 Legion.
These fit great, the 3.6" slide still has some room between it and the end of the holster but if that bothers a guy you can simply trim the holster end.
I now run the X300 Turbo with Antimatter wing as well, which is fantastic and the wing has fit fine into holsters as well. Even my Safariland ALS holsters work with my comped P320s.
So, it may be, that you can simply run your normal holster but let the comp hang out a little or just order a holster for a longer model FN 509 and be fine, if there are longer models...
Do you want a leather thumb break style holster? Falco holsters may do one for you, just ask them about accommodating your comp by adding length to the holster or just leaving the end open. And they do have a customization option, so it may not be an issue.
Looks like they also have a Kydex Level II that is the hood style, again, you'd need to mention them about adding length or leaving it open to accommodate the compensator. They look to have these in a pancake style OWB or one that would accommodate the safariland attachments like the QLS.
This post is long enough, I'll stop now, and yes, I love running a comp, I've done ported, my 2011 is V6 ported, one of my P320s was magnaported, I'll take compensated.
