Pipercross ones only cause a 1BHP drop vs no filters (allegedly) and cost around £18.
Neil at Sigma recommends them over undertank ones which can apparently damp the air pulses
http://www.sigmaperformance.com/996biptech.html 13BHP regained by removing the big foam filter!!!
I had an undertank one, and replaced it with the pipercross ones. Noticeable difference, and a change in the intake note as well.
But then again, some prefer the undertank filters.
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