This is awful and the converter doesn't need replaced. Hmm...I'll be in Westminster MD in a few weeks - I'll fix it for $500, deal?
You can type in P0420 in google and read for years about this debacle of an issue.
Catalytic converters last the life of the vehicle unless something abnormal compromises them (very rare)...so slow down. 1980's Subaru's are still passing maryland emissions just fine on their original converters (if they haven't rusted off!). Subaru converter failure is extremely rare and about the last resort, not the first.
First - you should tell us exactly what codes lit up.
Although I'll just tell you that one of them was the P0420 code that likely popped it's ugly head.
This issue is due to some very tight tolerances (too tight) in the monitoring algorithms. All sorts of miniscule things can set off the P0420 code...the tolerances are just plain stupid and not worthy of the real world - they're impractical and don't even effect emissions exactly. The vehicle I can guarantee you would pass Maryland emissions sniffer tests...they just won't do it with the CEL on, but it would physically pass - i'd bet $1,000 on it...well more than that, because I'm right.
The $5 fix is the smartest thing to do. But first you need to make sure there are no exhaust leaks, the engine is in proper tune (have plugs/wires ever been replaced?), and the O2 sensors aren't the culprit, and it is indeed the 0420 animal.
If you're not up to the $5 fix or reading about this debacle of an issue then pay Subaru to do the test that have for this....make sure they don't just tell you to buy a converter - make sure they do the proper computer/smoke/oscilloscope test - the real deal, whatever it is called you can probably google that too. they have a very rigorous way of narrowing down this issue rather than just throw $2,000 at this thing.
aftermarket converters can cause the P0420 code to light up and also may rust out quicker depending how cheap you go. Heck - i've seen new Subaru converters still have the code comeback - because it's never the code. Often replacing the converter fixes it only because they also replaced some leaking gaskets, a lazy O2 sensor, or heat shields in the process or the new converter just barely made up for some other issue...when in reality the converter wasnt' the problem at all.
correlation does not mean causation...
in the end the $5 O2 extender is what intelligent rational, practical people are doing. do it.