This makes for an interesting discussion. I suppose the barrel distortion in the Hyperion could explain my results. What I'll need to do next is check out the drift at different distances from the center and look for variations.

It's not at all obvious to the casual eye that it's there, and certainly doesn't seem to visually distort any extended features. I'm also curious about what angular magnification distortion might be present in either of the Hyperion or Nagler. From my visual impressions so far, I think I'm willing to put up with a subtle barrel distortion in order to pick up a wider true field of view. I guess it wouldn't be such a good choice for astrometry, though.
What surprises me most is how people seem to think that apparent field of view is such an obvious and objective characteristic. It's not for me. Peripheral vision is a funky thing, and in wide field eyepieces, that comes into play at the edge of the field. It's actually hard for me, at times, to see a difference in the apparent surface area of the field between the Hyperion and the Nagler. I have to look closely and move my eye around to confirm that the Nagler is wider.