These are direct bolt on air cleaners made by S&S for S&S. I use the Muscle variety. More than adequate and they do add noise. The original black hockey puck was primarily designed to curb the noise of the intake as that is all taken into consideration for being street legal.

http://www.sscycle.com/go-fast-showcase/motorcycle-induction/stealth-air-cleaner-kits/

What's your mileage? Who cares. Is it practical? See #1. What happens when it rains? You get wet.