Ralliart P0420 fault code

[Hubdubs]

Hi all
I own a 09 ralliart sport back
Car is completely stock.
I have this re-occurring check engine light which throws fault code P0420.
This apparently points to a faulty cat converter.
I called my local mitsu dealer and I almost passed out when they told me the repair cost.
I'm after some advice. As I am building a house I want to go the cheaper option if possible.
Where would you start?
Aftermarket parts?
Is it safe to go to any exhaust joint and get a high flow cat cut in? Or will that lead to more fault codes.
Could it be my O2 sensors, or do they throw another code?

Thanks for any advice

 

[MOR31]

rear o2 or cat problem

 

[t-nol]

An aftermarket cat can only improve flow, the sensors really only tell you if it's blocked, so it won't hurt it to have better flow. Try wreckers if you're on a budget, then do the aftermarket option.
Be easier to replace sensor first though to try that option.