take this as you want, and depends on what you want from the car but i have 3inch from the turbo back, highflow cat and a fujitsubo 3.5inch catback and it is awesome sound setup.
It gurgles when it idles, but when driving normally it is very quiet compared to all other exhausts around, but it sounds even better when i push the right peddle down hard and it just sounds mean, its like a lion roaring.
It is due to the metal fujitsubo uses in their exhausts, it quietens down the engine noise enough to a gurgle and when cruiseing it isnt very loud at all even though it is pretty much unrestricted. If you still rather the sound of power when you want it and you have the money go for fujitsubo.
Hks and apexi exhausts seem to make the cars sound loud all the time, and some times its a bad thing....
i dont know who sells fujitsubos other than shift performance in lansvale, mine was done in japan before i got the car though....
There must be other exhausts like this because not all skylines with aftermarket exhausts are excessivley loud.
Got a friend with a NA supra with a hks cat back and it is rediculously loud (sounds awesome), but loud all the time...