Sorry for the attitude. I take out my frustration for "the tuner scene" on new people sometimes because you haven't made the mistakes yet that others have. Coming on here and asking questions, learning, and trying to be respectful of those around you is already a welcome change of pace though so thank you for that. We pride ourselves around here on being nice folks, even to those with catless exhausts, glad I didn't scare you off.
You should look for an inspection shop that also works on sports cars. Many of them will care that you're catted but won't care if it's aftermarket. Generally outside of California you can use a catted aftermarket exhaust unless the shop is really draconian. Some shops won't care about anything...
Also, cats do fail but they're also made of precious metals. Failed cats are still worth about $100. It's not throw away money.
Another point about enviornment stuff is that cats are a double edged sword. They remove nitrous oxide which all but elimited the threat of acid rain and they remove carbon monoxide (smog) which made living in downtown cities far less of a health hazard. They have, however, significantly raised the CO2 footprint of our cars (they convert CO->CO2, adding oxygen from the air). So we're killing ourselves more gradually I guess.
2015 STi: Still Stock
04 STi, 04 WRX: SOLD