I've never known any of the Nexus 7, 8, or Alfine 8 speed hubs to have excess drag that affected real world performance in any meaningful way. Most of the drag, as far as I can tell, is simply the constant of the seals doing their thing. Sure there is a bit of loss from grease & meshing gears. But it's only a couple of watts different than a conventional derailleur drivetrain. Easily comparable to rolling resistance or dirty & cross-chained gear selection. Nice tires can mitigate any/all hub inefficiency as far as I'm concerned and get you back to square zero in the efficiency department.
In short, the spin down test doesn't really tell you much. There really isn't much of a comparison between a pound & a ½ of rim & tire compared to the momentum of 200 pounds of bike and rider. Put your concerns on to things you can control.
That being said: Proper bearing adjustment & the occasional servicing isn't going to hurt. The bearings should be just tight enough there is no detectable play at the rim edge; no more.