I agree that a well designed airbox will add power, with the use of Helmholtz resonance tuning - the problem is that it tends to be concentrated in a very narrow RPM range. I've often seen a secondary wave tuning affect at a much lower RPM, which also helps the power down below, but this also often results in a dip in VE between these two peaks.
Still, overall, a properly designed airbox of sufficient volume and rigidity and intake area/design can and does perform better than an "open air" design, and with significantly less noise.
This doesn't mean that there aren't specific cases where the airbox is so poorly suited to it's motor, or is so restrictive, that chopping it up and opening it to become a quasi open-air design doesn't improve the performance - clearly there are cases where this results in better breathing, but that doesn't mean that a completely different and better designed boc can't improve the power even further on that same motor!
Additionally, the use of a sealed rigid airbox allows the use of the cold and ram-air intakes, which further improve efficiency - something that open intakes or open airboxes generally can't do, due to their location.
The latest Japanese sportbike boxes are much better designed than the old bikes of 20 years ago - and take up a huge amount of real estate, compared to the older designs, requiring all sorts of changes to motorcycle packaging.
This is a subject that I'm real interested in, and will be pursuing further as soon as I can find a decent deal on the extensive dyno tests needed to test the effects of alternative volumn airboxes on my Duc.