Help with Exhaust choice

Before I list the models I have found available for my car I would like to ask what is the advantage of adding a cat-back exhaust system to your car. I understand the basic principles of getting the exhaust fumes out faster but it seems as though if you don't replace the headers there would be no point or advantage in replacing the remainder of the exhaust system as the headers can only move the gases out so fast, rendering the cat-back system useless??? Please correct me if I am wrong, this is how a computer works, if you have a slow bus and fast memory the fast memory is essentially somewhat wasted.

Here my options though (very limited):
1. Apex World Sport Series
2. Racer Design Cat back

I'm not in to the loud exhaust simply performance gains.

Thanks in advance.

well your kinda right and kinda wrong. If you do headers it will help your exhaust flow better (wheter its stock or aftermarket) but the exhuast on factory cars is bit choking to a motor replacing it does help. the Apexi WS is nice really quiet also. But you can think of it like breathing your intake lets more air in headers and exhaust let more out. doing them all makes them work together better. But just parts do help.

In my eyes, you are shifting your bottleneck. It is dependent on the car, but one of the components in your exhaust currently requires the highest exhaust velocity to achieve the proper flow rate. My advice would be to start by replacing that piece, moving on to the new bottleneck, and so on. If you know your pipe diameters and runs, you can calculate where the highest velocity in your system is, so that you will know which component to replace first.