A plugged cat will run worse at higher rpm if it is bad enough to affect idle.
Common to check for vacuum leaks by plugging all fittings you can temporarily and then testing idle quality. Once the plugins are isolated as good then you go to gaskets and/or possible cracking somewhere to leak unmetered air.
See if the front O2 sensor is stuck low below .5 volt at warmed up idle. Indicative of a vacuum leak.