How much of the inner mech rotates when coasting? Is the drag/spin down the same in every gear? I read on the internet (and if you see it on the internet, it must be true) that 5th gear was best for coasting (?).

I'd expect the quieter Alfine version to have MORE drag, since it uses a roller clutch instead of pawls.

...and make sure your cup&cone axle bearing aren't too tight, as they often are from the factory.