It is easy for some else to say about putting the foot down and demanding things when you are not experienced to any of these. I have been in this situation before with my old lexus LS400. I didn not have $3600 bucks to replace the CATs so I took the car back and replace just one cat with dealer parts (install some where else with labor of $50.) I see a little if different with the acceleration after replace it. But the code came back up few weeks later, I keep reset the code and expect some other engine codes appear. One year later, my car was sudden shook violently, I realized the problem was the second CAT and time to replace them. Rightway, I purchased OEM part and replaced it. Now the freaking engine code is still on, I just recently swap the O2 sensor (left to right) and see if the code will change, it seems to solve the problem, but I did not bother to replace the O2 sensor, I am still driving the car without fixing the problem I know O2 sensor won't hurt the car.

I think the best way to approach the problem before replacing the cat is trying to check the O2 sensor first by replacing. It's cheapper that way than replacing the CATs