We hear all the time that there is no real impact on moving the CATS downstream from stock conditions which is a total myth

As we see below with a C6 Z06 how the PCM measures the CAT temps as in this case reporting around 1,350 degrees

Moving CATs downstream where the exhaust temps are cooler only has a negative effects then on the O2 sensors and the onboard tests the PCM does.

Also shows in fact the PCM reports when DTC errors tripped and if/when someone cleared such codes so GM or EPA smog testers can if need be tell if owner erased DTCs before car is examined as in this case you see the owner had cleared all DTCs 7 miles ago.