i also have a 2.5 inch cat into a 4 inch canon
i get it aswell
its around 3500 rpm and stops about 4000
however if i give it the beans it goes all the way untill 7000
i will take a video and youtube it for example tonight if im bored enough and post it.
i have been told that most NA skylines will get it and it something to do with the resonator.
p.s i recon it sounds tough when it goes to 7grand just getting faster n faster.. its something different.