Ok i have just been quoted £180 ($355) for a custom backbox and £280 ($555) for a custom Cat back exhaust system from powerflow here in the UK. Any one know what difference fitting a full system will do compared to just a back box? more power? Would it be worth spending the extra £100 to get the full system or would it not make a lot of difference and would be better off spent on an induction kit or decat pipe? You opinions would be grateful.

Its on a stock 1999 1.8 Honda Civic VTI-S DOHC Vtec 169bhp by the way.
CheersAlex