From a deleted multipost thread masscommuter asked:
masscommuter said:
Is it worth replacing the catalytic converter in pre -'05 engines?
I wonder - at what cost?
The answer to your question would be no, don't replace the CATs.
There are several possible explanations as to a specific type of _concurrent_ CAT failure that Trunkout was also experiencing that _could_ explain his stated significant improvement. But without first hand inspection of the CAT we'll never know for sure. A P0420 _almost_ never indicates a restricted CAT (although it could). If so the complaint from behind the wheel would rarely be a MIL, loss of power _and_ still be driveable.
Subtrafuge's reply is the most commonly correct in this situation. All the possible interactions of learned PCM "values". An answer to Trunkout's experience is beyond simple qualification. You'd have to do an extensive "research" project to know.
Is there a difference in 00-05 CATs :?: Maybe, you'd have to check the part numbers. Better to compare 02-05 since there were some changes after 00-01 that _may_ require a different PN. (an extra hole for another O2 sensor is one that comes to mind)
Do the 05 CATs "flow" better as Trunkout has been led to believe :?: Again maybe. The PN's would be the only "armchair" answer.
If the 05 CATs do flow better and you replace it for an otherwise good 00-04 CAT, would you see a significant overall power / MPG improvement :?: IMO little or none. There isn't _one_ "simple" thing you can do for as dramatic an improvement. Such improvements require something more complex e.g. a turbocharger. Or that the car is so poorly designed to begin with there's no place else to go _but_ up :!:

As you already know, not so with an Insight.
HTH!
[edit typo, sentence structure]