It's been proven many times and also depends if you have the Race or regular replacement filter on many brands like Pipercrosse, BMC, etc..not just K&N.
It doesn't have anything to do with them and business. K&N is a filter than can be cleaned and recharged (as they like to say) when it's dirty. You can't do that with OEM so that is K&N's claim to fame, some vehicles you do get a hp increase but depending on bike/conditions, etc..the Hayabusa isn't one of them. They also can cause mass air meters to fail and have rough idle issues to but that again depends on the make/model and application..
I've also seen people claim 5hp increase with a K&N but have yet to see anyone post anywhere near those gains.
For the cost, you are better off getting all short stacks for the airbox over a K&N if you want peak power gains...