Reviving this thread a little, as I've been researching this topic, online, today, a bit:
I think Thunder may be on to part of the equation. In a "ghetto" sort of way (his word, not mine.), he brought the factory restriction back AND straightened the airflow, so that now the air has to pass through the filter, change direction to the back of the filter housing, THEN do a 180 down the beer can sleeve, for a shot down the home stretch toward the carb. Many people online have mentioned how pods mess with the vacuum signal to the kidney-shaped post at the top of the CV carbs. Proper signal to this port is crucial to the vacuum slider's functionality, but it just may be that the Beer Can Trick solves this issue as well.
One of the jobs I've worked on involved utilization of a flow straightener that almost identical to this basic design. Only minor difference was the presence of numerous holes at the end of the tunnel where the air has to make that 180. They help in making the flow around the bend, easier. something that certainly can be incorporated into the beer can sleeve.
Obviously, if you are looking for HP payoffs, this MAY not be the method for you. BUT, if you are looking to streamline your bike, or just want that "cool pod" look (Don't ask me! I don't give a shit about image!
), this just may be the ticket.