Is it?Standard air box is too restrictive.
Just look at the letter box the air has to get through, the pressure drop must be phenominal. Got to let the engine breath with minimum pressure losses both on induction and exhaust.
There is a bit of shades of grey I imagine. From what I understand the best filtration material is the paper type. The fabric types are next and the foam types the worse.I do love the induction noise but if it was potentially harmful to the car i wouldn't do it .