Turbo backwards exhaust (£1000 approx) will see improvements to horsepower as thats bolted from the turbo. see horsepwer gains from this.
Catalitic converter back (or catback) is at the bottom of the Stock factory downpipe, around where the centre consol is in the front, in my case one of the silencers removed to get it the slightly harsher exhaust note and releaves some back pressure in the exhaust giving the engine less grief to chuck out the exhaust gases out da back of the car......... or something like that