Thanks for taking the time to test, it's going to help. I'm planning to adjust my future sensitometry tests in response to this thread, I'll add an exposure without a 6-stop filter in the well. To date, I've "never" seen a shoulder so I want to see it too.
sun of sand,
I agree halation is going to wreak havoc - good example you mention. But halation occurs on exposure, not development.
Highlights burning, would depend on the shoulder and I still don't think PeterB is even within 2-stops of the shoulder (a guess - must... see... sensitometry... I really must begin testing). But even if not shouldering... Burning highlights will certainly be less "convenient" because PeterB may not feel like giving 80 second burns (another guess). PeterB did the right thing for highlights, by developing a little less he will have shorter burn times in the highlights than if he had developed normal.
I also feel that switching to a speed-reducing developer - for the purpose of this shot - wouldn't have been wise. It would be the right approach ONLY IF the developer is already in his repertoire (If he already knows what he might get - like the shape of the curve and the quality of the grain of a different developer like Rodinol or Caffeinol) AND if that is the look that he knows would work for this shot. That's what I think Ansel Adams did for Moonrise, Hernandez, New Mexico - he already knew it would work for the shot.
Just a side point, I'm not looking for a magic bullet, I've already found a working combination - and "slight" overexposure is an important part of my magic bullet.