2005 Outbacks have a very specific issue - I believe part of a recall. I have heard of numerous 2005 models having their catalytic converters replaced for free via Subaru....and there's an ECM reflash as well, though that might be for later models, not sure but just check into it, Subaru should know well the details.
Here's a guy with a 2005 that had new converters installed:
Recall Notice WVM23
What they said is true - there are a few "catalytic converter" related codes, we need to know which one it is.
Let's for a moment ignore this recall issue and assume this is due to a P0420 code while I'm here just in case you need to be prepared:
That being said.... in more general terms (specific issue aside) Catalytic converter replacement is stupid (I'm putting it lightly) and one of the biggest debacles of the automotive industry right now - due to the P0420 code which is a joke.
If this is due to a P0420 - your converter is working fine (*see below) and would pass any state emissions sniff test if they'd perform it (though they won't with the check engine light on).
*Converter failure is only likely if the engine has seen serious abuse or massive engine issues. Highly unlikely. (or it's part of that recall deal noted above).
There’s a $5 fix that should be done first, so far it’s worked every time I’ve done it and to every friend I’ve recommended it too. That is standard P0420 practice for all newer vehicles due to the scope of this issue. They now sell the part all over ebay ready to go, a few years ago you sort of had to make your own from a part at the auto parts store (though it was very easy).
New sensor isn't a bad idea, that generally doesn't fix it and doesn't affect engine performance (that sensor isn't even used by the ECU routines), but they're not that expensive, easy to change and a lot simpler than exhaust replacement.
Any leaks need to be sealed up.
Yes catalytic converters are expensive. Buy a used one or aftermarket, but cheap stuff can rust quickly. I just priced a complete Bosal exhaust from the headers all the way to the muffler (including catalytic converter) for $225 shipped to my door from Rockauto.com. Only $100 for the converter section...i'll buy 3 of those if i have to before the insanely priced Subaru units...I ended up buying a complete Subaru system from Colorado (no rust) for $175 shipped to me from a board member.
A shop can easily and inexpensively (but you need a reasonable shop that doesn't charge a lot – often times they can charge a good bit) install any exhaust - it's just pipe really, not much to it. I've welded and installed my own, exhaust/muffler shops are set up for it and can do it very quickly. But - a check engine light is possible if it’s far from stock (Size and such). They should be able to mimic the original though.