I was looking around a lot of the VWR cars at Oulton and I noticed nearly all of the were running an OEM airbox with maybe a crude intake scoop at best - but then, they're all restricted to power output, so do they need the benefit?
I'm not convinced all the ones on the market actually offer any benefit, it takes a lot of thought and intelligent design to achieve something effective, and that is often too cost prohibitive to make it salable... who'd spunk that kind of money on the GruppeM one without absolute, independant proof it was worth the money?