I have both an FNS-40 and an FNS-40L. I also have an EFK Firedragon conversion barrel for the FNS-40.
- Conversion barrels are adjusted so that the headspace is correct, and the extractor works as it should. The difference in rim size is typically not an issue. (I've owned a bunch of conversion barrels that let me move from .40 to 9 or .357 to 9. I recently picked up a .40 conversion barrel that I can run in my Glock 37 (which is .45 GAP) -- it works perfectly, and that's a big difference!
- The only obvious difference between the FNS-40 barrel and the FNS-40L barrel is the length of the barrels. While I've not fired the gun with the long-slide barrel installed in the FNS-40 slide, it chambers and ejects round just like the 40 barrel and it did not seem to bind. The COMPACT FNS-9c and FNS-40C barrels use a totally different design than the larger versions and that is probably where the misinformation about 40 and 40L barrels being quite different came from. You won't gain anything of significance from that longer barrel except a little extra velocity -- and it'll just look strange.
- You can use the .40 mags when shooting 9mm if you don't want to shoot hollow-point. I found that the open mouth of most hollow-point rounds tend to catch on the feed ramp when using the .40 mags. I shot using the .40 mags for a while and just bought two 9mm mags.
- I did not change recoil springs.
- The barrel openings on the front of the slides are, of course, different. The 9mm conversion barrel is modified to fit like a .40 barrel in the FNS-40. The opening of the FNS-40L barrel opening is larger than the FNS-40, because the barrel must tilt at a sharper angle. I don't know if this will cause barrel or slide "wear" issues, over time.
- Most barrels bought for service pistols from gun makers ARE drop-ins. (That seems to be the case with Glock, FNH, S&W, etc.) That is certainly the case with the EFK Firedragon barrel. My EFK barrel was drop-in and it's at least as accurate as the factory barrel. If you buy a barrel from Bar-Sto, one of the best after-market barrel makers, the barrel, dropped in, will typically shoot at least as well as a factory barrel without gunsmith fitting, but if fitted will likely shoot better. (People often let Bar-Sto do the fitting.)
I would have bought a conversion barrel for my Long Slide when I ordered the barrel for the FNS-40, but they weren't available, then. They are now. If you can't wait, go ahead and order the long-slide barrel and use it. You could also have a local gunsmith cut it back to the proper length and put a target crown on it. (You might ask EFK Firedragon about this -- they will talk with you...)
Another option is that I'll swap my FNS-40>9 conversion barrel with you for the Long-Slide version. (Mine has maybe 100 rounds through it -- its like new.) I'll even pay the small difference in price. That might speed things up a bit.
All that said, I think you're better off just being patient and getting on the EFK Firedragon waiting list. They're a good company to work with. (I got one of their barrels for my Browning Hi Power and it's a great barrel, too.)