A good custom exhaust centre should be albe to do this, I did something very similar on my old Seat Ibiza FR TDi, ran a new pipe back from the cat with branching into 2 pipes with 4.5in oval tips. Took a good few hours to do but as there was no silencers it keeps the price down. All in it cost me £200 and I've been quotted the same for the GT! although I think that could go up as the spare wheel well will need modified. Unfortunately for you i'm in Scotland (
www.motorsportworld.co.uk), sure there will be somewhere near you that could do it tho, probably not at this good a price tho.