For that level of 'noise' you're going to have to remove the cats and the silencers, and replace them with straight-through pipes - however if you're in the UK you may want to have some method of refitting the cats for the MOT.
Oh, and expect to be pulled by the BIB quite often and possibly be given an ASBO on the car - get two of them in a year and you lose the car, with no redress or right of appeal whatsoever.