I wonder if anyone did a dyno test comparison, sock off, sock on. How can the sock restrict the water and yet get more flow per square inch? the surface area of the air filter must be ten times the usable surface area of the sock.
K&N has sold what they call the DryCharger for many years, and it's basically the same thing as a rain sock in Harley terminology. I assume the parts are similar if not identical, and the K&N part is a thin polyester material that's treated with a water repellent chemical. They claim it adds "very little restriction", but I've never seen them actually quantify "little".
If you plan to have your bike on a dyno in the near future why not be the guinea pig and have back to back runs made, with and without. Like I said before, I doubt you will see a truly significant difference. And don't forget, even without the cover the air flow changes over time as the filter becomes dirty, so you don't get maximum airflow under all conditions anyway.
Jerry