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.

I cant really gove you an answer,but what I can give you is a way to a solution, that is you have to find the anglde that you relate to or peaks your interest. A good paper is one that people get drawn into because it reaches them ln some way.As for me WW11 to me, I think of the holocaust and the effect it had on the survivors, their families and those who stood by and did nothing until it was too late.