Well, few years back I spent over a grand getting a Suzuki I had to make some proper power, with another 800quids worth of race pipe it made over 100bhp, 40 up on stock. It had cams, head work, cdi, carbs rebuilt, the list went on.
The folk at TTs left the airbox just changing the filter, cut the inlet up a bit. I said 'I expected them to change the airbox or junk it entirely' and they replied 'that is done only for drag bikes and "show over go", stuff'. 'Worst thing you can do for performance junking the airbox'.