I just got my X a few days ago and I have been exploring options about improving fuel economy. I have been thinking about getting a drop in K&N, or even the FIPK. Have there been any concrete test results that show a gain in fuel economy with either option. Even some experiences would be nice. I would, of course, also like to increase power as well. Are there any dyno results out that show some concrete gains?

Also my X is still under warranty would either of these two options void said warranty?

Any help that could be given would be great. Any experiences would be appreciated as well.

Thanks guys.

Mileage depends on your driving habits. But there are people out here, including myself that have seen a raise in mpg (1-2 mpg). The Air filters will not void your warranty. Unless the dealer supplies free filters,(oil/air)this will not void your warranty.
There was a court case involving this very subject. It was determined that if the dealer does not supply free filters, the dealer can not void your warranty because you used aftermarket filters. Magnuson-Moss Act.
Click
http://www.mrinkman.com/Warranty.htm

I just installed an intake system by Airaid on my S/C. Overall, I'm pleased with the results. Though I don't think there's been any significant increase in HP as some state. To me there is a definate improvement in throttle response. The jury is still out on the MPG improvements, but I'm optimistic that will improve.

Overall, I bought the Airaid system because they make a model specifically for the X and the $$. Airaid is the most economical - that I've found anyway. If you search this site (and you should - lots of good info here) you'll find that the intakes (K&N, Airaid & AEM...others) pretty much produce the same results.

FWTW the local parts guy at Nissan told me that the NISMO intake is actually furnished to NISMO by AEM - they're the same, but you pay more for the NISMO product.