I would imagine the biggest restriction in an exhaust of my MY00 UK Turbo is the downpipe cat. After the cat is it un important what the rest of the exhuast consists of.

For example my MY00 UK PPP has no centre cat, the down pipe cat remains and has the Prodrive back box. Does it really matter which back box I fit whether its a Prodrive or standard STi 5 for power production, would i even notice a differance in drivability?

 

removing the cat from your downpipe will increase the power. But changing the boxbox salso can have a slight increase. Id do your downpipe mate, backbox is more for sound and show.