ok, so lets put this simply on what i have read
The air filters that are put on a car by a manufacturer are restrictive due to regulations set by governments/EU, so this will impede the flow of air to the engine, hence marginally lower peformance acheived.
So either drilling the air box and removing the throat as '6 said, or putting a performance filter on, will increase the air flow to the engine, and if im not mistaken, this will improve the performance of the engine, but will not have a dramatic effect to it, or the top end speed of gears.
This to me seems a bit obvious because more airflow = more oxygen = better burn of the fuel!!!!

And '6 this is the most calm argument i have ever seen you partake in!!!