Well as for the aftermarket cat being legal (it depends on the laws of Idaho). In Florida (and most states to my knowledge) it is legal to use an aftermarket cat ONLY when your stock cat is damaged/broken and needs replacing.
Your problem is you don't have one. So I am not sure, but an aftermarket cat should pass emissions (I assume you have testing). However, I would take the car back to the dealer and demand he put the car back to stock form w/cat (a subaru/oem exhaust/cat, I would check your downpipe to to make sure it is stock/has a cat) for free. B/c the law is on your side.
FYI: I edited my post in your thread, as I for some reason I put the Carolina State Atty General's name (Cooper). The Idaho State Atty General is Lawrence Wasden.
If I can be of anymore help let me know. I understand all this Cat (Catalist), car, and legal talk may be out of your comfort zone, so don't be afraid to PM me or post as myself and others on this board are here to help.