Cat Converter Problem in 2008 Prius II

My cat converter was stolen about 3 years ago and I had it replaced with after market kind. About a month ago the check engine light code came on and the OBE reader showed code P0420. About a day late it suddenly went off. Took the car to the dealer and the service manager (who ostensibly owns the same model) told me it's the cat. He told me these after market cats only last a few years. He also told me it won't hurt to drive the car as-is, just the emission quality will go down. I live in DC and they don't check emissions on a hybrid. Yesterday the P0420 code came on again. Thinking about replacing with a Walker or Magna flow. Don't want to spend $1000 on another converter given that the car has 175K mi on it and if these after market models only last a couple years why should I invest that kind of money in it?
Thoughts?
Anyone have concerns about driving with a degraded or bad cat? Does it hurt the car?
What about Magnaflow or Walker brand cats? Any experience with these?