What would cause catalyst system efficiency below threshold already had the catalytic converter and O2 senors replaced?

You have spent quite a bit of money and still have the problem. Obviously the converter and the O2 sensor did not need replacing. Take the car to a professional who actually knows what they are doing, and stop throwing parts at the problem.

You have spent quite a bit of money and still have the problem. Obviously the converter and the O2 sensor did not need replacing. Take the car to a professional who actually knows what they are doing, and stop throwing parts at the problem.