Some tend to opt for just the mid and rear and only swap out the Y if its damaged, doing the lot makes the system consistent and the OEM Y is renowned to fail over time, so from a fitting point of view it's easier to get done in one go...
They are both louder than the Nissan system and this is noticeable through the rev range, if you like the OEM sound you'll love the Cobra resonated system...
If you wanted a small jump in sound then the Milltek system we offer would fit the bill perfectly...