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That's a tough question to answer and the best one I can give is "well, it depends" The design of everything from the airbox that houses the filter, the intake, and the filter can effect how much air is restricted. Stock air filters tend to be made of a paper material that has microscopic pores in it to allow air through while blocking contaminants like dust and mice. Performance air filters have larger pores to allow for more airflow through the filter. To change out the shape of the filter, you usually have to change out the whole intake as well (like going from the square filter in a box to a cone filter.) The air filter type can only do so much without upgrading the rest of the intake to accommodate. If the pipe into the engine is too restrictive, it doesn't matter how big the filter is or how many filters you have- the engine can only get so much air through the intake.
The power gains versus amount of intake modification you do can vary widely from vehicle to vehicle. For example: just taking the cover off the air filter box on the MazdaSpeed 3 added something like 10whp according to the now defunct Sport Compact Car Magazine. While on the other hand, changing out the whole intake on the last generation Celica GT-S with a CAI only netted 2-3hp on the top end and resulted in a lot of low end bogging down when accelerating from a dead stop.
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