I'm with you all. I have a healthy appreciation for my limitations as a marksman. From what has been said so far, it sounds likely that the limitation in my situation will be my marksmanship capabilities and not the ethical maximum shooting range of the .30-06. Thus, the job now is to find out -- from shooting my hunting rifle from field position, not a concrete bench -- what my limits are.
Just as an aside, it seems one of the on-line sites I consulted was hosted by the Colorado Department of Wildlife and proposed 250 yards as the maximum ethical range for the .30-06. I would have thought that was maybe fixed a bit by their apprehension for the quality of average marksmanship, but they had no problem saying the .338 Win Mag was good out to 400 yards. Still, I suppose it is still possible they are building in skepticism about average marksmanship: maybe the idea is even a bad shot at 400 yards with a .338 Win Mag may effectively anchor an elk and allow for a killing follow-up shot.