Ive read where some have used FMC's to use fiber cable in place of a long run of Cat type cable.
Looking for feedback on this.
I have a 10M Cat 8 to my server and wonder if there would be an improvement by going the fiber optic route.
Depends on the nature of the device at the end. If you have an EtherREGEN at the "destination" end, then, it won't be an issue as its designed to mitigate the problems of longer runs of copper Ethernet into audio devices, e.g. network bridges or streamers.
If it isn't an EtherREGEN, then, very likely, Yes, you would hear an improvement. The optical cable will remove the effect of RMI and EF and high-source leakage impedance current that copper Ethernet is susceptible to.
If you use an FMC, the most important thing is not to use an FMC with a switch-mode power supply for the FMC at the destination end where you'd be converting back into copper Ethernet, as this will add high-source leakage impedance current back into the of the copper Ethernet going is going into the Ethernet receiver of the destination networked device.
Good clocks, really help, too, so if you don't need a switch at the destination end, minimally you should use a Sonore OpticalModule (OM) as your FMC at the destination end. Better results will be obtained by using OMs at both the source end and the destination ends because the Crystek CCHD-575 clocks in OMs are significantly better than the super el cheapo clocks in generic or consumer FMCs e.g. tp-link, etc (note: these are the exact same clocks in the EtherREGEN because both were designed by the same Ethernet EE, John Swenson). The "cascade" and stack-up of phase noise and jitter from the crap clocks in generic devices e.g. NUCs, computers, routers and generic switches in the network chain is quite audible.
One of the advantages of optical for longer runs is that also that it's easier to route than copper Ethernet, which is why I use it.