I have recently swapped my MY00 Prodrive Backbox for a MY03 STi Prodrive backbox and it has made some subtle changes to the car and the noise it makes. Firstly the car feels a little more responsive and revs cleaner at the top end. Secondly the noise from the exhaust is a little louder and deeper.

Are there differences in the construction of these two boxes (apart from the larger tail pipe) that would increase performance/flow or is it all in my head?