I don't think that's necessarily true. I think Nick Symmonds' 800m ability would be quite well predicted off of his all-out 400m/1500m combination, but I see him as having a "fall-off" point at much over a mile.
I bet Jonathan Johnson would be the same. Since his 800m ability is superior to his 400m, it looks like he's getting stronger as the distance increases, so you'd probably get a quick 5k prediction (though we all know that wouldn't happen).
Meaning there are other factors than just two numbers involved in determining what someone can run at a particular distance.
However, it does imply that, while maintaining your top-end speed, when you improved your 800m ability you made a comparable (and predictable) improvement in the marathon, and vice versa.
So as you increase speed, in the neighborhood of WR mile velocity pace, does that go up or down (one effect from more/less time spent running, one from lower/higher velocity). I assume it would be similar for the 400m, so do you think MJ could have run 41.5 with a proper rabbit?
I'm sure you've posted it somewhere, but do you have a link to the formula? Or could you at least explain how it works (what degree polynomial)? Thanks.