One of the first things most people think of when considering air-flow restrictions is the air filter, but in most cases, it's not the air filter that's the problem, but the air box or the air filter housing. The best way to check whether the air filter or the air box is restricting air flow, is to use a water manometer to check air flow at various RPMs before the air box, before the air filter, and between the air filter and the mass air-flow (MAF) meter.
Should either the air filter or air box be restricting air flow, replacing the air box and air filter with a high-flow cone air filter, such as a K&N air filter, is the best option. But you must box the cone air filter to ensure that hot under hood air does not enter the air intake. On naturally aspirated engines, under hood temperatures can be as much a 30º above the ambient air temperature, and on forced induction engines, it can be as high as 50º above the ambient air temperature! As you know by now, higher air temperatures result in lower air density and, consequently, lower power output. Higher air temperatures can also lead to detonation, especially on forced induction engines.
Another popular school of thought suggests that an air filter reduces horsepower, and that your engine will make more horsepower if you run without an air filter. This I call pretzel logic – it makes perfect sense but is fatally flawed! The correctly sized air filter will have a slight effect on air flow, but only slight, while running without an air filter will have a major effect on engine life and engine power! The purpose of an air filter is to prevent dust and dirt from getting into the combustion chamber and grinding away at the piston rings and cylinder bore, not to mention your turbocharger and supercharger. Pretty soon the slight power gain obtained from ditching the air filter will be lost through worn piston rings and seals. Do the smart thing, always use an air filter. It's not there for show and it's not optional!