Replacing the back box will not give any performance gains as the standard system is very efficient. Even with a cat back you may only gain 1-2bhp.
Cat back systems usually remove the rear box completely leaving you with only 2 siilencers in the system which gives a nice increase in sound but its not in your face. I have a mogoose cat back system and its no louder than the standard exhaust when plodding around infact its quite understated at full chat too.
I would save up and get a blue flame exhaust as they have a good rep and are under £300