You can get low lean angles without putting your knee down. I ride often with plain biker jeans, therefore I don't put my knee on the tar. All you really need to do is to shift your hips while keeping your knee tucked in. Knee dragging is very cool but its not a requirement for road riding. The basic principle behind knee dragging is to shift the center of gravity of bike + rider, so that you can go through a corner with LESS lean angle. Less lean means more grip therefore faster through a corner on the same line.
Let me put this another way. I go and practice at my local track often. Sometimes I wear full leathers, sometimes just biker jeans and a leather jacket. Irrelevant of what I wear my lap times are consistent. I am not slower when I don't drag my knees. The reason can be 2 things - either I am slow anyway, or dragging my knee is not making a real difference. As for the slow I don't think so - my lap times are among the top 10% of riders on that track. Do I go faster if I wear full leathers? Yes, by a few seconds. The reason is that because my knees are protected I go faster which causes my knees to drag. It is an indicator of my lean angle, and that is what makes it a very useful tool. When my knees scrape I know I am near max lean. When my elbow touches tar I know I am on the edge of disaster, perhaps even slightly over.
That is what you see in the pictures above - those guys are pushing laws of physics very hard and are perhaps just over the edge. But then they ride in MotoGP and we don't.
On a standard Busa you will start rubbing the sides of your boots (or footpegs) and your exhausts before you get enough lean angle, so you will still have some chicken strips. To get to the max you will need rearsets and aftermarket pipes. I had to raise the rear by 1 inch to stop scraping the fairing and stator/clutch covers.