I studied a bit. So, we have 2 cats. But my assumption is that the first cat takes all the beating and is probably 90% likely to be the one with the issue. If it were me, I would consider buying the $371 cat from rock auto. It comes complete to the manifold. However, I'm not really trusting that manifold, I'd have to eyeball the stock manifold to see how similar it is to the one pictured on rockauto.
If I lived in a location that did not do emissions testing, I would simply pull the stock manifold and bad catalyst off, and replace it with one of the "performance" headers that have been discussed in these forums. Not for the "performance" aspect of it, but as a way to eliminate the catalyst. In littlemoule's case, if that catalyst is plugging, and he put a header on it with no cat (but a bung for 02 sensors), I think he would see rather large performance gains.
The downside is, I think he'd be getting cat efficiency codes. Which I could live with if I KNEW that the codes were simply telling me that the catalyst that I DON'T HAVE is not working effectively ... and if the car was running good.
littlemoule - A lot of the Magnaflow cats I saw looked like the 2nd cat in our system. So, you'd have to be sure you're getting the correct cat for fitment reasons. The Magnaflow stuff looked like decent or better quality though. The first cat (on the 2020 which is what I looked at) appears to bolt into place. No need for welding. But gonna need good impact tools to remove those bolts.
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View my fuel log 2020 Mirage ES 1.2 manual: 42.4 mpg (US) ... 18.0 km/L ... 5.6 L/100 km ... 50.9 mpg (Imp)