oh, well it would operate better if you still had a functioning o2 sensor threaded into the test pipe.

the ECU takes some feedback from the secondary o2 sensor for fueling calculation, it isn't exclusively for just catalytic operation check. generally when there is a need to eliminate the CEL from a test pipe of high flow cat, a spacer is used to withdraw the o2 sensor from direct exhaust stream, but still keep its operation in check.

You may have kept the CEL from coming on, but not sure if your emissions readiness monitor for the o2 would go ready, which could prevent you from passing a more modern emissions test.