Well, I have no stake in accuracy the runworks calculator. That might have been the first time I looked at it. I just found it from a Bing search. But it seems somewhat plausible to me. Your formula is just that the benefit is half of what you would lose going uphill?
Just to see if your formula works for me, what would you calculate the level ground pace to be for 4:47 pace on a 11 percent downhill? I know what it feels like to me as a good downhill runner. What about 5:00 pace at 8 percent downhill?
I was saying that altitude would probably have little effect, but that was when I was overestimating the benefit and thinking of my runs on what turn out to be 11 percent downhills. I agree that altitude would have an slow down effect. Of course, it would also have a slight benefit of decreased wind resistance - you would be going 13 mph. Probably a net slow down effect, but not as much as you are estimating.
As for the any human being able to sustain that quad damage, I've said that I believe, for good downhillers, it isn't going to cause much damage. I don't get quad damage in an untrained-for-downhill condition doing 8-12% grade downhills at 5 flat to 4:45 pace for 6 miles, so quite a bit steeper than 6.5%. For 99% of runners, they will really, really feel the quad damage that you speak of in that kind of effort. For me, I feel no quad damage at all after that kind of downhill, even when untrained/untoughened for downhills. I don't feel damage during the downhill, immediately afterwards, or days later. I wasn't always like that - something clicked for me about 2010 (after running a lot since 1986, lifetime miles now is probably around 70,000 miles), and my downhill form just became super easy. I could easily see sustaining that for at least 1:30 -1:45 in an untrained-for-downhills condition, because that's about all I have when I do long hard/steady workouts (or races for that matter) and I'm a poor marathoner. But that would be 18 miles already. Maybe I'll try it when or if I ever go to Maui again. Again, I am not immune to quad damage. It's just that when the pace is near 5 flat or faster, I don't get it. I only get quad damage when the fastest I can sustain is slower due to the need to brake (not due to it not being steep enough).
Anyway, even if 2:17 effort will give you 2:00:00 on Haleakala, ignoring altitude effects, it would be easier than 2:17 effort because you would be running that effort for 17 minutes less time. That could also keep you under the elapsed time beyond which many people bonk.