IMHO, based on what I've read on several different forums, there are gains to be made from some standard type bikes by modifying the airbox and inserting a hi flo filter. By standard, I mean FZ1s, Zrexs, Super Dukes. Most standard bike engines are detuned, so why try to wring every pony out of them from the factory?
The RR type bikes are a different beast. I've had 2 RR types, a 929 (which I gave to my son) and an 07 600RR. Those bikes are set up to a higher level of performance, and every componant is a tuned part of the system. I left mine stock.
I used a holesaw to cut a hole in the side of the snorkel on my 919. When I put my hand over the hole, I could feel air going in when I revved it. More than likely, a feel good thing, but I will say that the bike runs pretty damm good.
Unrelated? I've also read of guys cutting holes in the airboxes of 4 stroke dirtbikes like the CRFs and WRs and YZs. My KTM XC200W doesn't really even have an airbox.