2002 grand prix se 3.1 - performance or better cat

[sicayan]

i have code p0420, replaced front o2 sensor, erased code, and back to p0420 after 70 miles. since i am going to have to buy a new one anyway, i would like to get a better one, a direct fit and possibly a new muffler, don't know about the resonator. is this just a waste with a 3.1 or can i get something descent for a little bit more than what i would end up paying for an oem ($184, not including muffler, just direct fit cat) from shucks or napa?

thank you,