The catalytic converters reduce unburned hydrocarbons and reduce oxides of nitrogen. No amount of 'tuning' will every achieve that. So your statement on reducing emissions by removing the catalyst is provably false.

People under 60 have no idea how crappy the air quality was in the late 1960 through 1980. And it really didn't improve much until the mid 1990s, when most if the old cars finally aged out and left the road.