Yep its illegal (but so is speeding, being drunk and disorderly in public and talking on your mobile phone in the car- but we all do those things dont we ) but i run straight thru 3" piping on my WRX and i've been over the pits 5 times in 4 years and only the last time i was told its too loud. Once the cat issue was picked up by a younger bloke working there and he just told me "you have a special dump pipe with a cat in it. ok!"

It was loud and mine didnt really sound like a wrx in the higher rev's