Hi Ammo
As others have said, the “quiet “ R500 airbox does take a perceptible edge off the performance; can’t put numbers on it but you can certainly feel it from the driver’s seat.
You re-built my engine last Aug as a Raceco 250 after it had ingested a bit of fractured air trumpet. Since getting it back I’ve run it with the “quiet” airbox, without an airbox just the pipercross filter, and with the C400 airbox. From comparative noise tests the “quiet” airbox is no quieter than the C400 airbox, and I’ve had the Raceco 250 engine with Caterham roller barrels, std de-catted Caterham primarys and Raceco exhaust mapped by Steve Greenald at 269 bhp, well pleased
I’ll send you the dyno plot.
Cheers
Mike