The Nismo exhaust (which is the same on both versions of the car) definitely sounds better than the standard exhaust. But, the improvement is fairly small. If the standard exhaust is 2/10 in terms of volume, I'd say the Nismo is 4/10.
While I doubt you'd have problems fitting the short tails, I do wonder if they might look a bit 'lost'. If it helps, here are two pictures which show the difference made by the Mk1 Nismo's rear diffuser (although the Mk2 Nismo seems to have a different shaped diffuser):
My previous 370 with Stillen exhaust - note that the tips are quite prominent.
My current car with Stillen exhaust. The angles of the pictures don't help but 'in the metal' the Stillen's fairly hefty tips look much less prominent with the Nismo diffuser.
I think the Nismo looks best with an exhaust which has larger tips such as the Cobra, Stillen or Ark, but the Mk2 diffuser might not have the same effect as shown above.
There are a number of reputable traders on here who can supply exhausts - Octet/Torqen, Cougar Stores, Tarmac Sportz (based in Derby) for example.
All that said, you could always fit the short tails and then sell them on the forum if you're not happy with them.
Hope that helps.