Ethernet to SPF converters. What's your experience?

I have had the following chain for 4 years: router/modem->fiber-optic->Melco S100 switch->fiber-optic->Lumin U1/X1

I have had a more complex variation too in another system.

In a resolving system the difference between copper LAN and fiber-optic is not subtle in favor of fiber-optic - the result mostly of a lower noise floor. BTW I am not talking only in systems that are expensive, you don't need to spend mega bucks to get high resolution, despite what a lot of snooty audiophiles might claim.

My 2 cents.

Best thing to do is to try. Just remember

1. all these switch mode power supplies used with network switches and converters throw noise back into AC line

2. Routers, converters and switches generate a fair amount of RFI and EMI

Connect these gadgets to a completely different circuit from your audio system and keep them away from cables and electronics. Run a longer Ethernet cable or a fiber cable, whatever your streamer accepts to keep all this noise away. 
 

In some cases you might get an improvement, in others you can make it worse. I would start with the basic router to streamer connection, live with that for few days then add a switch. Listen for another few and introduce the fiber conversion. Quick A/B is never a good idea and will most likely steer you wrong  

Before having a hi-fi Ethernet switch, I had a basic fiber converter, which ran via an RJ-45 to my LUMIN U1 mini. Then I bought the Hi-Fi Ethernet Switch, and the system worked much better, all over RJ-45. One lazy day, I tried installing a fiber converter on the long stretch from my server, but the results weren't good: short dynamic range and extension. Finally, I decided to buy an SFP card for my server and connect it with OM3 fiber optics to my Hi-Fi Ethernet switch. Then, a Cat8 OCC+Ag to the LUMIN, and the change was incredibly positive: more dynamics, micro-detail, and texture in the bass. As @Kariosman said, the change for a small investment has been great.

I have had experiences with FMC devices.

But first - the first response to the OP was about clocks.  I am interested in this subject.  It looks like @sls883 has a Gustard DDC.  I am curious to know if @sls883 is using the I2S input on the DAC.  It is my understanding that a good way to use an external clock in this situation is to connect the external clock to the DDC and then connect the DDC to the DAC with an HDMI cable and the clock signal is 'transmitted' through the HDMI cable to the DAC.  I don't understand " I have the clock connected to the N18pro, the U18 ddc, and the X-30 dac". Does the clock have 3 outputs?

Now on to FMCs:  My experience was initially with a Sonore microRendu which has ethernet in and usb out.  I used two "cheap" FMCs in series to provide galvanic isolation (I think I'm using the correct term).  I used the wall wart power supply in the "upstream" FMC and a Sonore LPS in the FMC closer to the microRendu.  I used a cheap fiber cable.  For me there was an improvement - more "relaxed". less digital "glare".  Not a big investment, the most expensive item was the LPS.

The next step was to replace the cheap "down stream" FMC with an upgrade one from Sonore, which also included an upgraded SFP module.  Similar improvement as above, but maybe more subtle.

Next I replaced the microRendu with an opticalRendu which has fiber in and usb out.  This eliminated the upstream FMC.  This was a big improvement, not inexpensive.

Then, over at the Audiophile Style forum I read a long thread about using a Direct Attach Cable (also unfortunately referred to as a "DAC") which replaces the fiber cable and ELIMINATES the galvanic isolation!  I bought an inexpensive DAC and I like it better!  So I'm currently streaming through an FMC but I'm not using any fiber cable in the chain.  Pretty crazy.... 

as @audphile1 has said, you just need to try this stuff and see what sounds better to you, in your room with your system.  It seems that 100 hours of "break in" is a good idea.  And always buy stuff that is returnable. 

I think cheaper is better, but where the AC adapter is matters.  If it's near your equipment or sharing an AC line, keep it on the "dirty" side of a power conditioner and consider an aftermarket supply like these.  They won't make the network connection better but they will reduce the amount of noise in your AC line.