EXHAUST'S

what exhaust's have you changed to,i currently have supersprint single middle pipe without a box & jap back box,don't know if the car has any more power but with this set up the car feel's quicker when accelerating,down side it has a bit of a drone at 2-3000 revs,thinking of getting milltek cat's next,so is it worth getting the cat's done,has anybody got any proof of improvements made by there exhaust changes, & where is the best (cheapest) place to buy the cat's from