I battled catalyst efficiency codes on the last Toyota I owned. Even after replacing catalytic converters. What I learned was Toyota programs high catalyst efficiency thresholds. You can get a p0420 and the catalytic converter can be 70 - 90% still good. So unless you have driven with the light on for thousands of miles, you have been consuming a quart of oil every 500 miles for years, have removed the catalytic converter and cut it open, I suspect it's not clogged. And your problem with performance and catalyst efficiency is somewhere else. Check your fuel trim. Vacuum leaks, weak (front not rear) O2 sensors, dirty TB and MAF, plugs, all can contribute to a P0420.