The catalyser seems to be the most misunderstood component in the exhaust judging by the antics of some owners.
It's a common strategy to remove the substrate - this VX220 N/A owner is in the middle of doing this on the downpipe bolted to the manifold, hoping that he will release some bhp. Reducing the back-pressure innit! This one is a metal substrate and it's tough to hack it out. Unfortunately, where you really need the gas-speed to be at its highest, he's created a large void which slows it right down - not the best feature to encourage scavenging at the exhaust port.
All the Toyota-engined cars use the same body (mantle) and ceramic substrate for all the different motors - 4 & 6 cylinder.
Here is what the internals look like on the V6 motors which have them fitted -
You can see it's a very restrictive honeycomb - 600cpsi (cells per square inch).
The log manifolds (headers) on the V6 use these UTC's (Ultra Thin Catalysts) to help the car pass some very strict worldwide emissions tests.
The whole design is a serious block on power output and with increasing power outputs on each new V6 model, they are under a lot of supercharged stress. For instance - don't even think about retaining these manifolds on a TVS1900 install!