http://www.dieselpowermag.com/tech/general/0610dp_aftermarket_air_filters/index.html

Notice they are comparing new filters to new filters
(apples to apples)
and don't play the typical magazine trick
of getting an original equipment air filter really dirty on purpose,
then comparing it to a new aftermarket filter.
If you are willing to play that deceptive trick
it is easy to show nearly any hp gain the author wants.