IMO the difference between a stock air filter and an aftermarket one (referring to drop in type) is that the performance of an aftermarket air filter doesn't deteriorate as much as the stock air filter.
So if you're comparing a stock air filter and an aftermarket one with both BRAND NEW, the performance is the same. Only the stock one deteriorates faster.
Owners of eight and twelve cylinder engined BMW’s manufactured between 2001 and 2009 are advised to bring in their cars for BMW Malaysia’s technical service campaign...