The idea some have that the standard airbox is somehow restrictive and not up to the job, is laughable.

Size and shape of the airbox intake, and airbox internally is all calculated, airbox design is complex stuff to get the most out of the engine over a wide range of RPM.

If you were doing a turbo conversion where the engine suddenly demands lots more airflow, then yes it would need rethinking and would need to be larger.

Some airbox design info:

http://www.saltmine.org.uk/randy/airboxdesign.html

"The airbox used to be just an intake silencer and a place to put the air filter. Now it's much more than that, so read on before you gut or toss your box. Just as is being done on new cars and motorcycles, snowmobile airboxes and their intakes are being built as resonant systems. When the airbox is resonating strongly, driven by the engine's suction pulses, its rapid internal pressure fluctuation covers a range of plus and minus 10-15%. This is just like the resonance of a bottle when you hum into it. If your engine's intake events run in step with the positive side of this resonance, it's just like getting a 10-15% supercharge boost for free. That's worth having. And what if you modify your engine, raising its peak-power rpm beyond the range of the airbox resonant frequency? There is a simple relationship you can use to alter airbox frequency by changing the length and/or diameter of the airbox intake pipe(s). That's worth having.

Any hi-fi enthusiast knows that woofer enclosures work best when the resonant frequency of the enclosure is nicely centred on the speaker's response range. The enclosure usually consists of a sealed volume with the speaker installed in one of its walls, and an opening, called a reflex port, cut into the enclosure. A resonant system consists of a mass, which vibrates back and forth against the restraint of something flexible, like a spring, with an excitatory force to drive it. In the case of the speaker enclosure, the mass is the air in and within one diameter's distance of the reflex port. The spring is the compressible air inside the enclosure. The system is set into vibration by the amplifier, driving the speaker cone back and forth as a piston.

In the case of an engine's intake airbox, the mass is the air in the airbox inlet pipe(s). The "spring" is the compressibility of the air in the box. The excitatory force - a very powerful one - is the endless sequence of strong engine intake suction pulses from the carburettors. The airbox must not have any significant leaks, as the throttled, back-and-forth airflow through them acts like a hand on a vibrating bell (anyone who's ever tried to play low notes on a valved wind instrument knows what a killer leakage is). The airbox inlet pipe is usually made with a smooth bellmouth on either end to reduce flow losses. Carburettors or throttle bodies must likewise seal positively to the box. When a system like this gets to humming, the pressure inside it vibrates rapidly plus and minus 10-15% of atmospheric pressure. In fact, the humming is so powerful that in many cases a sub-resonator is placed near the atmosphere end of the inlet, to prevent radiation of this powerful honking sound to the outside."

Some more air filter info to make you think twice before spending money:

http://mymiata.paladinmicro.com/K&NComments.htm

http://www.max-boost.co.uk/max-boost/internet_articles/Re%20NO%20K&N%20NEVER%20on%20a%20944%20NEVER%20-%20The%20Porsche%20924,%20944,%20968%20&%20Turbos%20Message%20Board.htm