Hi Steevie,
Changing the box to a cone will make several differences. The main difference will be that your engine will breath properly which will result in a little more performance.
It will also help to allow the engine to run a little cooler. It helps in the sound department too!!
As far as fuel consumption is concerned, I think that it will make little or no difference at all. The little difference that it does make will be in your favour. If the engine is performing a lot better, it will require less throttle.
There are several filters on the market but the best one has to be the K&N. It was tested extensively along side HKS, Nismo, Apexi, Blitz etc by experts on behalf of Banzai Magazine.
Conclusion: K&N allows a far superior air flow, trapping even the most smallest of particles and utilising them to tap bigger ones which means that it better the dirtier it gets!!!!
Add to that, it is advised by K&N not to clean it for 70000 miles, you will be better off with one of them.
Happy hunting buddy, you will not be disappointed.
El