A couple of points to note,
Firstly the ECU post cat sensor is there to measure a multitude of emission factors which must all fit within the OEM factory parameters.
So the gas flow that our cat gives is hugely greater than that of the OEM unit which has to negotiate two 600 cell monoliths. The ECU is therefore communicating that something has gone outside of what essentially are fairly wide settings indicating that something needs investigating on what it thinks is the OEM cat, this can happen within days or even months or sometimes never...
Most Stage 2 maps can remove this code. The alternative is to install a lambda spacer to lift the probe out of the gas flow...
The most important factor is the efficiency of the cat at conforming to current emission legislation which ours does handsomely.
The balance is remaining road legal and maximizing the power potential...
Please feel free to call me for further information.
Cheers
Nige