The airbox is on the intake side of the turbo. Its the other side of the turbo that needs to be sealed properly to ensure boost is maximised and not lost. On the intake side can therefore be modified to maximise flow - provided the increased flow is efficient. This means accessing cold air from outside the engine bay, rather than sucking up the hot stuff from within.
The MAF will pick up on the higher air flow and the ECU will adjust fuel to maintain the prescribed mixture. The combined effect means greater air and fuel = more power
The subject mod would have you cut a hole in the face of the airbox that faces the opening to the inner guard and facing the louvres in the guard liner. This gives access to the ambient air and supplements the factory ducting. The louvres are angled in a way that minimises crap sprayed in - the balance is up to the air filter.
I have done this mod on mine and found the induction noise barely affected - its very modest and barely detectable. I haven't picked up any power gains either, but 3-4 hp would not be detectable from 'seat of the pans' in any case. Others have found gains on a dyno though.
hope this helps.
