Just gonna chuck in my 2 cents.

Exhausts are designed to get emissions out of a car. However as the euro is for "luxury" it is definatly not the best designed system.

Headers is where it all starts to get the fumes out of the engine bay. If you do nothing to the headers and get the best of a high flow cat and cat back exhaust you will see some gain but nowhere near as much when headers are installed.

If you make the engine get rid of all the exhaust gases efficiently you will be able to gain more power.

A stock header is usually 4-1 where the 4 is only really used due to the engine design and is all put into 1 pipe asap to save costs. With these and the combination of an engines cylinders not firing at the exact time and the exhaust gases not being extracted quickly enough will enable pressure to build up (this is al happening before the cat). So if you ensure the fumes are being extracted efficiently and reduce pressure you will see gains.

Now if you change your cat to a highflow cat it will make a difference and will get the fumes out of the car make them able to be omitted to the atmospher legally all a high flow cat does is does it quicker.

The B-Pipe or catback exhaust has almost nothing to do with an exhaust system except to get the fumes out you can design it anyway you like and can be customised like a mofo and is usually why is cheaper there is no science to it. The only thing that you need to keep in mind is consisty. If you have headers that the pipe drops into a 3" pipe you would not get a anything lower then 3" due to pressure will be backed up and will be pointless in getting headers. If you have a 2" exhaust you can go bigger however if there is no pressure then that is also bad however a bit bigger can reduce pressure and create a more free flowing exhaust which will enable a slight performance gain.

Now the experience with my exhaust. I have 4-2-1 headers the 1 being a 3" pipe going into a highflow cat. I then have a 3" cat back with dual exhausts and 2 resonators inbetween. Mine is nowhere near loud and dont want a loud exhaust. It is designed to increase power with the 4-2-1 headers while still being quiet.

The summary. If you dont get the exhaust fumes out of the engine in the first place the other mods wont do to much.