post cat o2 has nothing to do with engine performance or mileage, its only purpose is to monitor the converter.

 

This is the reason aftermarket high flow cats only have a two year warranty.. they don't put anywhere as much material in the cat as opposed to a stock cat.. They will be used up by that period and you'll need to purchase another... That also why a stock cat is so expensive too.. they have more of the precious materials inside that are used to "clean" up the exhaust.. If you want trouble free then a stock one would be best bet as it should last much longer (assuming the car is in good tune) ..

 

well i'll just probably end up making it a straight through

 

Actually you have a few options.

1 - emulator - this will work fine for your engine and model year.
2 - code reader - you can get these for 70 to 100 bucks and it will read the code and you can delete them. Anytime a P0420 comes up, just delete it.
3 - HP Tuners - get this equipment for 400 bucks and you can simply remove the code completely from the lineup and not worry about it at all. You can also tune your vehicle as well if you are going for performance in the future.