VN V6 commodore factory cat made by "AC Delco" with this brand stamped on it are the popular ones. You can pick them up for great prices too from a Holden spares place. They are known as the highest flowing factory cat that was made. Obviously just make sure its not shattered or anything inside it before you buy it. I think I got one for $70 or something. Two separate exhaust places told me that these flow as good as the aftermarket $200-$300 "performance" ones and if you can source one, then definitely grab hold of it.
I think there may be a bit of BS with all this "high flow" cat stuff. I'm not certain, but I reckon the diameter has to be the main thing, as the interior tunnel designs don't appear (just by eye) to be greatly different between models. I have always wondered whether its a standard fear they put into car enthusiasts to make them think they are losing power unless they buy an aftermarket "high flow cat".
That said, I destroyed 2 separate high flow cats (one was an expensive aftermarket one, cost me about $240 in 1999), the other was the VN cat. They both ended up shattering and rattling to pieces. Most likely due to the drastically different mixtures that blue monster was running compared to say a factory micra, plus it was pushing a lot of gas out the tail pipe through these things.. But I'm sure being 2.5" capacity they would have been more than able to handle the pressure from blue monster, they are designed for V6/V8s etc. I'd say it was more to do with mixtures and or vibrations/harshness or a combination.. My 2.5" exhaust ended up being reasonably rigid just due to the tight fit and how it needed to hang and be positioned.
I was advised to go for a metal high flow cat. These are immune to shattering and flow as good as you can get - probably equal to a well flowing normal cat. After that was installed, no problems whatsoever. That said, I sold the car a coupla months after it was installed, so I never owned it for long enough to know, but I'd say being metal you wouldn't have many issues. It was a bit more expensive, I think $300-$400, but after all the investment and custom stuff blue monster had, it was worth it just to avoid future hassles.
If I was ever to start over again with a custom turbo project, I would just immediately install a metal cat and be done with it. Those aftermarket "high flow" cars are too expensive to risk one shattering on you.