[caverman]

It will mostly depend if you live in an area where they are required in order to get your car inspected. Obviously if they are required you'll need to get some High Flow (HF) cats when you buy headers. If they are required then you'll need to make the decision whether or not to spend the extra money.

Some people have mentioned that without cats (often called off-road connection pipes) that your car will have more of a gas/exhaust smell. That will be from the outside of the car. You shouldn't smell anything inside the car. I've heard others mention that it will really depend on your tune as well. If the tune is more on the rich side (which is safer but slower) then you'll smell it more, if not then you probably won't smell anything.

HF cats are easy to add after the fact but they usually cost more if you don't buy them as a package deal when buying headers.

You can double check when you get your quote for headers but 99% of the time when you order for a 5th Gen they'll be a 3" inlet with a 2.5" outlet to hook up to your exhaust. You have to specifically order a full 3" exhaust to get it 3" in/out. So, don't be too concerned with that.

The cats bolt up or slip on to the collector part of the header. Below is a pic of some KOOKS headers and HF cats. With KOOKS they have a flange that bolts them together. However, ones like ARH have a slip on type cat with a clamp to hold them on.

The first pic is Headers and HF cats. The second is just a HF cat. The third it an off-road connection pipe if you decide to go that route.