sc400ny2va

It's me again the one with the funny vibration and hesitation only at 1500-200k rpms between 40-45 mph. I took my car to my mechanic this morning to get the ATF flush like you guys suggested but my mechanic advised me against it since the car has 160k on it and i havent changed it in 100k miles he told me to leave it be,because putting new fluid in might cause it to slip and since im having no trouble with it beside it being diry and smelling bad to leave it alone. He said my funny vibration at that speed is restriction from my cats starting to go bad. He explained its a big job on SC's becuase we have two. Anybody do this to their car before? Outcome was worth it? Is there a way to by pass this by taking the cats out? Any advice would be much appreciated. He told me to leave them alone for now since its only causing me a problem at that certain speed and rpm's. Any advice? Should i go for second opinion? What about the ATF should i really leave it alone?

Thanks guys