my old system used to have a center muffler and a rear muffler, first i took of the center one cause it wasnt loud enough, that made a difference, but not enough for passangers to even notice it...
finnally i pulled of the rear muffler and replaced it with a smalish 3 inch cannon muffler, with very little baffling material (whatever its called).
Its definatly alot louder, but the problem with skylines (moreso ones with stock turbos), is that they are impossible to make loud, i got the dude at the exhaust shop to start it up with the system off and it still wasn't all that loud...
improvment in power is very minimal, maby a couple of kws?, but more than likely, you wont notice it.