Kind of agree with the above about Custom stuff,
I've a Longlife on mine, had it fitted at mates rates for £200, chose the sound, tailpipe, the lot and I've been complemented on it un-countable times! The peformance of the car feels free-er at the top end too with no low down loss of torque.
However it does depend on the fitter, the guy I had was excellent and had been producing custom exhaust for years, some in the Powerflow / Longlife franchise are utter guff, so do your research and you'll be fine.
Anyone who knows me personally knows how much I spend on and Cherish my GTA, however £700 for a backbox seems ludicrous. Yes, yes it may have been designed for the car but is there an extra benefit for blowing £500 notes more!!