Dont knock out the inside of the cat or put a staight though pipe in it, its a waste of time for a few reasons,
1. The exhaust gases smell like s**t
2. huge, huge fines if you get caught

So yeah get a high flow one as people said, but with my experiances with cheap rubbish high flow cats you still get a bad smell out the exhaust even after it has got hot and set in. think its to do with the materials used inside it.

One shop was trying to make you buy more, the other was giving you good advice.

If the stock cat is good, keep it, as said they flow very well. If you're doing a turbo conversion, or your cat is f**k, then get a new hi-flow one.

I got my exhaust done recently, and was told to keep the cat if it's good. After fitting, they told me the cat was in excellent condition.

Any exhaust shop will do it for you. It involves cutting off the existing 2.25" flanges and welding on 2.5" ones.
As long as the cats in good condition, don't let them force you into buying another one.