I put forth Jakob Ingebrigsten: 55.5ish to 49.5 (est.) 51.03 PR and a 1:46 800
Seems like his training (high volume of threshold) helps you hold a faster pace relative to maximal speed for longer, even in the mid-distance races.
El G was probably not a great 400m athlete either: 54.9ish to 49 (est) also with a 1:46 800 (though a lot of us believe he was capable of closer to 1:43)
He is cited by Marius Bakken as doing some form of threshold training 11/12 days:
https://www.mariusbakken.com/training-corner/kenyan-training/kenyan-training-a-practical-guide.html