[cmiovino]

As far as the cat, as others have said here, just use either the stock front pipe, or get a high flow aftermarket cat. The stock front pipes are actually quite good and swapping them doesn't net much of a gain. The cat also keeps rasp down.

As far as "back pressure", what you really want in an NA exhaust is the ability for the exhaust gas to exit as freely and quickly as possible. This isn't just going with the largest pipe available.

Typically, stock exhausts are a hair small, but they're generally close to the right size because manufactures are looking for efficiency to balance power, fuel economy, etc. Usually speaking, going a bit larger helps power. Let's say from 2.25in inner diameter to 2.5".

The idea behind reducing "back pressure" as people say is just fluid dynamics. You want that exhaust pipe "the right size" to allow for the gasses to exit quickly. Not too big, not too small. And that is highly dependent on the engine size (IE V8 vs a boxer four cylinder like these), forced induction or NA, etc.