Glad to help.
Look at this chain of events:
A misguided mechanic convinces the owner to remove thermostat, and to "direct" the fan. The car never warms up and always run in Open Loop "rich" mode. The soot chokes the cat. The cat is removed. Performance improves, owner is happy, but not back to where it would have been if the buggering did not start with the thermostat in the first place.