So,, a gazillion dyno graphs posted all over the internet on a gazillion different make and model bikes, showing the increase in torque and horsepower from generously cutting open the airbox should, at least, prove that "most" bikes can benefit.
I think, Especially on a commuter bike like the FZ6, the airbox is a noise reducing tool. On a repli-racer, maybe the Helmholtz resonance is tuned more for performance but on a FZ6, Ninja 650, designed-for-the-masses commuter bike etc, etc,???
I know the engineers do an awesome job designing almost every aspect of our bikes. BUT, their #1 priority is "selling" bikes and to do that they MUST meet EPA (and the European equivalent) standards before they can even bring it to market,,,, so meeting EPA criteria is PRI-1,,, even on repli-racers. My point is, even though they are smart, good, great engineers their hands are tied to some degree and the air intake system and exhaust system are their best tools to meet these stringent noise and emissions regulations.
FWIW: If I were one of the "Air intake engineers", knowing that people are usually willing to modify their bikes, I would 1st design it so that it would meet EPA requirements but also so that the engine WOULD perform to it's potential once the private owner modified it,,, but that's just me.
This video isn't an FZ6 but you may find it interesting. Shows the dyno run for stock, slip-on and then the airbox mod.
[ame=http://www.youtube.com/watch?v=MQHMjnAScRI&feature=related]Leo Vince Install- Kawasaki Ninja 650r part3 - YouTube[/ame]