If you cut your airbox, then yes. Only because they are more resilent to outside forces of nature. If you mod your pipes with the cut airbox, then yes, you get a performance gain. How much? Don't know, but noticable, at least for me it is. Engine sounds better if you do either of the above.
My M750 sounds sweet when I crack the throttle. the bike sounds alive...sweet woosh of air...my monster breathes...my monster lives. I also have Staintune cans that allow the better airflow to leave the engine...if you do this kind of mod, it is a good idea to rejet your bike if it is carbed or remap your fuel injection. PCIII or something similar?? sorry I am a carb man : (at the moment)
It may not be necessary to open your airbox all the way..if you look down into your open airbox you will see that the air get funneled down before it gets consumed by your engine...that is where the bottleneck would be I think...so..as long as the hole made in the airbox lid is the same size, you will be getting the most airflow possible...Mine is wide open, but most likely does not need to be.
K&N filters are great..last longer(washable)..filter better, and have proven themselves on many a different types of motors.
There are many more post about opening up the airbox on your Duc...just search around - you will see.