Some Neons, I have read, had a sort of trick exhause system where the converter was also used as a muffler to save weight and cost (apparently). If the emissions system does not have enough "adjustability" (they are all able to adjust for ordinary wear) to get the mixture back to 14.7 to 1 WITHOUT the backpressure from the converter/muffler the milage may indeed go down. That's why the "test pipe" replacement is such an interesting approach, you can get one for a few bucks and judge the difference for yourself. Generally speaking, unless a converter fails due to corrosion, it is a problem elsewhere in the system which shows up because it CAUSED a clogged or overheated converter failure, an effect rather than a cause.