You never know what kills a cat.

The cars I WORKED ON didn't have cat pellets they were a honey comb solid core cats. They either broke up and plugged the muffler and closed off the exhaust.

Sense he won't tell what fuel brand he been running I would by some CHEVRON TECHRON to wash the intake valves from carbon build up. There is a difference in gasoline.

Run SHELL as they use nitrogen and I don't know what good that does.