That's great to hear the dealer got it covered for you! A genuine Subaru cat assembly is HUGE bucks.
I'm lucky to be in one of the "PZEV" States, so I'm covered for 15yrs/150Kmi.
I'd be curious to know if it was really the catalyst getting weak, or if the situation could have been corrected with new O2 sensor(s). I've seen where you can replace the downstream O2 sensor, it sees the proper O2 content drop and the light goes out.
As long as the downstream O2 sensor sees less O2 in the exhaust stream than the O2 sensor before the CC, you'll have no issues with P0420.