So I bought this fancypants Dash disc so I could run tubeless in the rear. It has, what I presume is, a fancypants hub. This hub may be the loudest hub on the planet while freewheeling. Now this noise comes at some cost, and the wheel clearly spins worse when freewheeling compared to my Zipp when I spin it by hand. This friction differential must be diminimus in terms of watts used, right?
Yes, it is diminimus. Your wheel is fine.