I remember someone dyno testing a BMW K100 motorcycle many years back (not turbo, of course). They dyno tested it with the regular filter, the regular filter with 80% of the surface covered with plastic wrap, and with no filter at all.

Guess what? No difference whatsoever.

Now, I am not an engine tuner by any measure; however, a friend who does have a lot of expertise in tuning engines once told me that reducing the air filter restriction on a NA engine almost always will do nothing for you unless you take other additional measure to increase airflow, e.g., an exhaust system which flows freer, head work/porting, etc. He said he has never seen a stock engine where the air filter restriction was the weak link or "choke point" in the airflow. Which makes sense, as if a simple improvement in the air filter would increase performance, surely the manufacturer would have already done this, as it is a very easy and inexpensive thing to do.