[kyleb]

WACR.
I don't believe you need a cat that suits a euro 3 configuration.
Just need a quality cat.
A quality cat is one with a high amount of the precious metals that do the 'magic'
to reduce emissions therefore they are more expensive.
I used one for a XF series falcon on my LS1 and got through first time.
Sambo is spot on. It's about proper placement of the cats and the airflow meter.
Also it comes down to who is doing the drivecycle test on the dyno.
If they short shift you get less emissions and vice versa.
Mine would come close to failing on the deceleration and we put that down to the Maf being too close to the throttle body.
This is all predicated on your tune being spot on and engine running right.
E.g No misfiring etc.
Cheers