ol1,
I have been a member on another site for a number of years and this subject came up a number of times, along with "best bang for your buck" types of performance modifications.
I believe the consensus was something like this: Car makers leave nothing "on the table" regarding performance. All the low hangin fruit has been picked.
Almost all "performance modifications" have a negative side at some point. For example regarding air filters: There is at some point noiser air flow into an engine, poorer filtering or improper flows over a given mass-flow sensor, etc. Fundamentally, there is little to be gained on the suction side of naturally-aspirated modern engines. The fact that a given air filter may sound like its "working better" by being noiser, is simply not consistent with better inlet airflow, (it is just noiser). I'm certain any number of converts will attest to better performance, but I believe they are unable to establish technical-clinical testing, by their personal bias, (after they pay money and install whatever - they simply want to believe it).
However, it is well-established that all manufacturers do make cars with relatively quiet exhausts to sell cars to the general public. And, there is some performance to be gained with replacing the exhaust with a freerer-flowing system. Almost always, the replacement is noiser and would not sell well to the general public. But in this case again the extra noise is also what the buyer wants of these systems (it just sounds more powerfull). So we now have a "win-win", with that particular owner, (perhaps until the car is to be sold).
I have not even touched on any warranty issues that might be violated.
IMHO, It may be much more cost-effective if you simply buy some K&N window stickers on eBay.
