what is the loudest exhaust you can buy that is still good for performance?

 

As far as volume is concerned the number of cats removed can have more impact than the choice of back box.

If you don't want any messing around come MOT time then get a Afterburner Vortex or Blitz NUR Spec with the standard downpipe left in place. But if you want it very loud, that downpipe needs replacing.