Early 06 Optima 4 cyl

The two problems could be related but it is unlikely. You would have to have a misfire condition caused by the ignition coil. That would send unburned fuel into the exhaust system where the catalytic converter would burn it. Too much fuel will cause it to overheat. Over a period of time the catalyst could melt and become plugged. The typical symptom is low power due to the restricted exhaust. In this case the converter just isn't doing what it's supposed to do anymore. That's what the rear O2 sensor is there to detect. Since the fault code refers to the converter's efficiency, don't expect to have to replace the sensor that made that report. A different type of fault code would have been set if the sensor itself had a problem.