Ethernet to SPF converters. What's your experience?

Yes definitely isolate the wallwart from audio. 

I’m running Nucleus One and a basic Trendnet Switch. Nucleus is plugged into a dedicated circuit 1, system (DAC and integrated) are on dedicated circuit 2. Switch is plugged into a completely different circuit. 
The Trendnet switch, to my surprise, smoothed out digital glare when used with Nucleus. I’m also running WireWorld Starlight 10 Twinax Ethernet cables from eero to switch and from switch to Nucleus. That was a very good improvement as well. The WW Ethernet cables are really good. 

Been using SFP's for years and will never go back. Only using quality converters from Edisoncreation and Sonore Optical Module Deluxe, with Afterdark LPS, 

Not all fmc created equal, I've had cheap generic, now use Sonore OpticalRendu which also serves streamer function, meaning Roon Endpoint. This separate from a much more expensive custom build streamer which now only does Roon Core duties. While this makes for more complex streaming chain, many have found two streamer setup preferable for Roon. I highly doubt on board fmc within routers or switches provides sound quality on par with dedicated high quality fmc. I'm also skeptical as to quality of stock fmc within streamers. SFP modules, also called transceivers are variable quality as well, I replaced stock with Finisar 1475 to good effect. Finally, quality of AC to these devices important as well, meaning quality lps, DC cables, power cable  and power conditioning to the lps. I've optimized every single component in streaming chain, it all matters. 

Aside the above, quality I2S implementation and connection to dac superior to usb input, this includes dacs with very nice usb boards. Replacing dac internal master clock with external clock can be nice upgrade.

I listened to my system for several hours yesterday with the fiber optic converter. I felt that the sound was softer and warmer.  I wasn't sold on the sound. 

This afternoon, I took it out of my system and could immediately tell the difference.  It sounds better without the converter.  At this point, I like things as they are, so I'm not sure that I want to try a better converter.  This converter was free, so I figured I'd give it a shot. 

While fiber links provides benefit of galvanic isolation, the optical–electrical conversion process in inexpensive converters may negatively adds jitter / phase noise.