In my humble opinion you would gain very little from the installation of the K&N filters. If you go with 2 separate filters you would have to figure out a new routing of your pcv valve, too. There have been several posts here on the usefulness of K&N filters. Most folks think they are a waste of money. And don't forget you have to clean and oil them with the "special" K&N products (more $$). There is more than enough things to maintain on these cars without adding to it. As Site says new air filters are available from Nissan. It has been my experience that with as few miles as I put on the car (2,000/year) they last a long time. Save your hard earned dough for other stuff. Once again, my $.02.
Cheers, mike