Mayday~ Catalyic Converters

I have a 2000 Nissan Sentra 1.8. There r 2 cat converters on the
exhaust header(2 cat in 1 piece). Recently one of them went bad (I already changed the O2 sensors).....

The shop told me, for about the same money, instead of fixing it, I could just buy a new race header and they'll install another cat, then extend the O2 sensors so I get more hp and also PASS the emission.

so....I did, went back to the shop 2 weeks later with a new header, but-- now they told me they r not 100% sure that it'll pass the emission. I'm pissed, but I gave them the car anyway.

However, few things I like to know, but I can't find the answer anywhere.

1. what's the difference of the catalyticMy question is, what's the difference between the catalyst converters on exhaust and the one under the car?

2. has anyone one replace the cat on the exhaust with a high-flow cat, then extend the O2 sensors? would that pass the emission?

3. what's the warranty on the exhaust system? some article says in the US all cars have 5 years warranty, some says it's by the federial - 7 years, 90k miles warranty.

i havent had that problem. since i want to get rid of the cats all together im going to install the off road y-pipe when i can find a shop that is willing to do it for a fee and not worry about being legal and stuff.

all i kno is that if they look like cats and u pass, they'll leave u aone no qurstions asked.