I would stay away from a K&N Filter. I like K&N for my carbureted motorcycle, but these filters need to be oiled to work correctly, and the oil can then end up on the MAF sensor which will cause a loss in power.
There are many stories from M3 guys who have done this, and then found a big gain in performance only after cleaning their MAF. At least those guys can get to their MAF easily.
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Kippis
986S
991S
Van Diemen RF97
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