Not sure why the warning C4244 hasn't come up before in this function, but here be some warning fixes:
tresult PLUGIN_API performEdit (Vst::ParamID paramID, Vst::ParamValue valueNormalized) override { const int index = getIndexOfParamID (paramID); if (index < 0) return kResultFalse; owner->sendParamChangeMessageToListeners (index, (float) valueNormalized); return owner->editController->setParamNormalized (paramID, (float) valueNormalized); }