A Cat-Back exhaust system or part exhaust system will, in most cases just replace the rear sections of your existing system. This modification leaves your existing catalytic converter and forward section pipework in place. The advantage by replacing most of the exhaust will be that you experience a more noticeable improvement in the sound emitted from the exhaust.
Replacing large sections of the original exhaust with a much improved flow from an aftermarket system, will, in most cases result in your car being given a power boost!
If the original catalytic converter (or GPF or DPF) in place means you dont have to worry about putting your car through its next MOT!