Why does the WRX have 3 cats and the STI has only 2?

Actually, Subaru claims the STi is LEV. However, it is a larger engine using higher octane fuel and higher boost pressures. They claim a cleaner and more efficient burn and don't need the third cat.

Hm, makes sense to a point. The octane requirement is not that significant in practice since most everyone runs both cars on the highest octane available. Boost pressure shouldn't have that much to do with it either, since boost isn't an issue at cold start when the pre-cat is most useful, and the max boost on a stock STi isn't that much higher than a stock WRX anyway. But the STi may have larger down- and mid-pipe cats and there are other engine differences that may allow for cleaner cold running.

I was thinking I read that the STi is an, " Ultra Low Emissions Vehicle" ULEV? I live in an area that doesn't even do emissions testing. So, it is really a moot point for me. Seems like the cat is the first thing a lot of folks like to take off. BlueSTi "Scary-Fast"

Don't forget about noise, on the STI it's less of a concern.

The STi has significantly more exhaust note than the STD WRX. Not that I'm a fan of loud exhausts. I hate them! But, the exhaust note on the the STi is not obnoxious or annoying. And it sounds nothing like the local Honda boys who have the big fart cannons on the back of their "hot-rods". BlueSTi "Scary-Fast"