The dumbest question is the one you didn't ask.
That said, I agree those wheels look pretty aerodynamic already. If you have time to test the duct tape mod, it would be nice to have actual data. But I'm guessing any benefit would be marginal.
Edit to add: Maybe you could use your road trip as a mpg test. Use the duct tape mod for half the trip, and remove it for the other half. See what happens.