That's a good question. If you are correct in saying that a junction between 2 DIFFERENT metals causes impedance related reflections, then one would assume that copper-on-copper would be the best outcome. That would not jive with my experience on the copper-on-copper connectors causing midrange ringing distortions. I honestly don't know why plated connectors work better than un-plated copper.
The only signal reflection problem I know of is related to digital signal cables only (SPDIF coax / HDMI) where shorter cable lengths (under 5 feet) will cause reflections back to signal origination and cause sound problems. I can hear this in short digital cables where the sound is smeared some.