It's interesting that using a 75 ohm cable with mismatched non-coaxially terminated RCA plugs (WBT 0144) that should likely result in unwanted signal reflections and square wave corruption, I still believe I hear a more solid and focused sound than with the Toslink optical connection that has no inherent mismatch.
Most of the articles and test reports I have read where they had the proper test equipment usually measured significantly higher jitter with the optical connection than with the coaxial. I guess this is to be expected when inserting another conversion layer in the signal path to convert electrical S/PDIF to optical and back again as well as the possible suspect quality of many inexpensive Toslink transceiver chipsets.
I'm not sure how much the impedance mismatch of RCA connectors affects the sound, but those who have sources and DACs with BNC, XLR and RCA connectors at least have the ability to do a rapid input switching A/B comparison to get some idea of real-world sound differences between BNC, XLR, regular RCA and "true" 75 ohm RCA, beyond electrical theory alone.
Steve