from what i've been told changing the exhaust,down pipe and fitting a decat will make the engine breath better, so because of this the boost is increased.(that is why some people experience fuel cuts) but how this helps when you have a remap i'm not quite sure, i can only guess that it will allow them to run the car at a higher boost level.
hopefully someone who knows what they are talking about wiil stumble on this thread and shed some light.