Through my experience w/ simota and k&N they are both reliable speed boosters especially the K&N..But regarding with the fuel efficiency, I doubt it would give you more KM with those filters.Just remember it also depends on how you behave w/ your GAS pedal.

Its like this:

-----------> more air intake = more speed

-----------> more speed = more GAS consumptions.

e.g; Turbo,Overupgrade Exhaust Manifold,Mufflers,Air Filters.etc.. This are some of the parts you will consider when it comes to speed.