A reason factories put airboxes on is that they reduce intake noise and keep rain out. An other is that modern bikes use intakes that use ram air and the boxes need to be pressurised to let the carbs work. Pressurised air may vary fuel level in carb. (Maybe not so modern bikes ie; fuel injection). I know that my Mikuni flatslides and K&N pods work very well on my 86 Gixxer. I have seen alot of discussion on needle clip position, jet sizes and mixture screw adjustments, BUT, I do not recall any on slide cutaway. I am talking about the angle of the bottom of the slide. Granted there may not be any available for these stock carbs. I know a cutaway is different on a 2 stroke vs a 4 stroke. I suggest reading Kevin Cameron's book "Sportbike Performance Handbook". He has a chapter on carbs. Good reading and an other source of info. I suppose one could increase the cutaway by machining.
That approach make have some merit, but I say that without trying that myself.
BTW, this has been a most interesting thread.
Steve