I can confirm that running a fiber cable from ones internet router (as in my case via a Ubiquity ethernet switch with a fiber uplink) to your audio room, already makes a substantial improvement over the 15 feet cat6 standard cable that I had earlier. It connected into my Innuos Mk2 Roon core/server via a fiber/ethernet converter that I used for this, which in turn was fed by an Sbooster Linear PS.
vs
Just converting the last 6ft makes limited sense, though.
Yea, the first case rings true for me as well which is why I suggested it, as relatively speaking it is a very cheap thing to try. Replacing a 15 ft cat6 run from my media room closet to the equipment rack by simply replacing the A side connection of my EtherREGEN+LPS1.2 with an sfp + fiber made a significant improvement. For my system I would say the improvement was close to what I heard by adding the EtherREGEN into the cat6 connection in the first place.
The Ethernet cable I run from my $100 fiber converter (which isn't cheap by the way...) to my Bricasti streamer costs about the same as all of the fiber hardware I purchased for my audio room, so compared to the prices of high quality copper ethernet cables, EtherREGENs, linear power supplies, master clocks, static ethernet filters, etc., it is a pretty cheap thing to try.
Obviously YMMV, but you may find like me a few feet of fiber just before your streamer actually makes a big difference and potentially saves you from a few extra audiophile network cleaning devices. As fiber becomes more ubiquitous in home networks one would assume audiophile components will continue to add native support for it (e.g. Lumin X1, EtherREGEN, ...). What will then happen to the audiphile side-market of network devices that scrub/clean noisy ethernet connections? On Audiophile Style there are already discussions about how different sfps and single mode vs multi mode fiber makes an audio difference, so maybe that will be the next area of networking tweaks (e.g. audiophile grade SFPs etc...).