I'm not tremendously cognizant about mechanical guitar issues, but this is what I think I see:

The neck is dead straight insfar as it's not at all twisted, but I think the outer end -- the headstock -- *may* have moved a fraction upward over the years, thanks to constant full string tension, so the strings are increasingly further from the frets as you move up the scale (towards the body, or towards the bridge). The increase in action is equal across all six strings.

Also, the luthier I consulted says he get the desired result by reseating the neck at a slightly diferent angle to the body -- which makes perfect geometric sense to me.

Now, maybe this guitar has *always* been set up like this. The action is not impossibly high, by any means, and it's certainly not an issue in first position and environs. However, further up the neck, I think it could stand improvement.

I've certainly played other instruments whose action is more consistent up and down the fretboard, and I figure my vintage Martin ought to be as good as them -- should certainly be *able* to be readjusted into similarly good shape!

About TUNING: I've had my doubts about being in tune all the way up and down the neck, especially back when I was starting to play more regularly a few years ago, after years of relative inactivity. I was especially alarmed that my harmonics seemed to be out of tune when relative tuning by "regular" fretted notes seemed right, and vice versa. However, at least some of that perception must have been due to my own diminished "ear," because now that I'm playing more frequently, I'm finding it easier to tune up correctly (or acceptably well, anyway). I do have to adjust tuning a tiny bit when capoing at fret five or so, or higher, but have always figured that must be "normal."

I had been meaning to post these questions by starting a new thread, but semi-consciously highjacked this one instead. Hope nobody minds.