If the P0420 is caused by a leak or the fuel injectors are not performing within spec, then it could lead to poor fuel economy, but if it's actually the cat performance that has deteriorated, it won't affect fuel economy.
I've punched out the honeycomb out of several cats when they physically failed, and then just ignored the p0420 CEL. Some people put a spacer in downstream O2 sensor to fool the computer into thinking everything is OK.
This has the best info I've stumbled across.