Yup... and all those engines turn some serious RPM to make their power so there's a lot of air velocity going into these carbs and the right bellmouth will make a small difference at power... Have you ever heard of that saying "every little bit counts"? This holds ture here... on the street you would barely notice the difference on a CG engine at 7 or 8 thousand rpm, but the same engine at 12000 rpm it may make a noticeable diference.
We used to do a mod on the CAG carbs which was far from a real bellmouth, but more of just tapering the opening and it was enough to gain a mph or 2 on top, but again, this engine spins at 13000 rpm and uses a 13mm carb.
I'm not saying it doesn't work, but just saying don't expect a big gain if you try it, it Will give some gain on top but even on a 40mph bike you might get it up to 40.5mph with this added into the mix. Anything that directs or smoothes out the airflow into an engine will give it some type of benefit and what you would most likely feel on the street would be to use a small carb like the NT,RT, or NT Speed, then add a 10" or longer tapered tube directly to the carb inlet, the taper would need to be somewhere close to 1.5 degrees so it can accelerate and cram in the air really well, but it also needs to be long enough to be effective at midrange, but unrestrictive enough to be invisible to the engine at higher rpm's since it will eventually reach a point where it can't cram in anymore and it becomes more of a hinderance at high rpm... but the same thing set up for high rpm would be really short like an inch or less with a similar taper angle and the inlet rolled so it can grab more air than just what's directly in front of it.
I'm not shutting down the idea completely because if you can find the right length and taper angle as well as rolling the inlet lip for a smoother inlet flow you could actually get a gain you would feel, the thing is most people at home don't have the tooling to do a nice even taper down a tube that's about a foot long and around 14mm inner dimeter.