When LegacyMtls are converted but have a Siger ComplexFresnel node in the Reflection Map slot, the Converter should address that, copy the settings to the internal PhysMtl and do away with the Siger node. Or at least make it a checkbox in the converter window.
And I felt that because this still addresses the Complex IOR settings, I kept this in the same thread. There should be a drop-down menu like the Siger plugin to have say 12 common presets (StainlessSteel, Aluminum, Titanium, Chrome/Chromium, Copper, Bronze, Gold, Cobalt, Iron, Nickel, Silver, Platinum) and then have the custom option available too. Right now, you need to pre-save a lot of PhysMtls to a library with your chosen Fresnel and to change it, it's more convoluted than just using the free Siger plugin.