Just received my K&N High- Flow Air Filter part #33-2326 from the K&N U.S site, I must say there is some sound improvements and possible performance improvements.
I believe there is ways of improving the intake via simple modifications like removing the charcoal filter (carbon filter) which is better in the garden ;) .
There is a carbon filter, which adsorbs the HC that accumulates in the intake system when the engine is stopped,
is used in the air cleaner case in order to reduce evaporative emissions. This filter is maintenance-free.
Along with the use of the air intake control system, an air intake control valve is provided on the air cleaner
case.