Not necessary to go 43.9 to go sub-1:40. A combo of 45-mid and 3:30 capability might be enough to go 48.7/50.2.
Coe's open 400 PR was 46.87 but on at least two occasions he ran 45.5/45.6 for relay legs (1979 European Cup/1981 Eng vs USA a few days before his 1:41) and demonstrated phenomenal kick on other occasions. Block starts were not his skill but in these relays he beat US national 400m finalists and Olympic 400m relay medalists he was matched up against head to head (e.g. Tozzi of Italy).
Important point... bear in mind that his 400 relay PRs were set during the same seasons he broke mile WRs, not in different seasons with different training. The fact he ran 1:41.73 with nobody remotely close after he blew by Konchellah before 500, and it was an early season run for him, and the race was run very late, partly because of the Lewis fake-WR in the 100, should also be taken into account. He was himself surprised by this result, suggesting he was not maxed out for the season. He later stated his aim for this race was to improve his early season marker with a ~1:43. Maybe as a result of this unexpectedly fast time he did not attempt the 800WR again that season, instead running 15s and miles and the 1k WR. No other 15/mile WR has run remotely as fast as Coe over 400 during the SAME SEASON he set the mid-d times.
Kipketer's 1997 WR run at Zurich was insane and he could have gone absolutely miles faster than 1:41.24 with a more realistic distribution of effort. That first 200 of 22.9/23.0 was ridiculous. Had he run 48.5 off 23-mid/25 even instead of 48.10 off 22.9/23.0 then he would have finished well under 1:41, likely he would still hold the WR. The difference between an even 48.5/6 and a wildly positive split 48.1 for the first lap of an 800 is monumental at this razor-edge level. I thought a WR effort was blown and I was quite surprised that he continued to move as well as he did in the home stretch. Nobody else has ever run that fast through 400 and finished even competently.
The whole uneven pace situation reminded me of Coe's 1981 1500 at Stockholm with that 52.4/1:49.1 opening i.e. a total waste of a supreme athlete in lifetime peak condition to run a monumental time..an opportunity that might never come again for that athlete.
His subsequent 1:41.11WR was run more evenly but I don't think he was quite at the same incredible peak as at Zurich.
Also his 1:42.67 indoor run in March indicated a potential for sub-1:41 that season. The differential for most athletes in this range is much greater than indicated. That time is still the indoor WR and has not been approached remotely. Therefore, it stands as a testament to what Kipketer could do with ideal pacing in his peak season and indicates his outdoor time was sub-optimal.
Kipketer had a fluency and turnover that even Rudisha could not match. I can't believe that Kipketer could not run 44-high at 400 at that time in an optimal race. However his 15 wasn't that great, in the Symmonds 3:34/5 class.
So from an athlete performance indicator pov, maybe ~45.5/3:30 will demonstrate the capacity. In the actual race, 73.5 at 600 is needed with the classic 26.4 close. So 48.5 at 400, even split.
Like a lot of posters above, I don't think anybody racing now is close to being able to do this. 1:42-high last season WL. Amos is the last athlete to break 1:42 and even that run was now five years ago. It was seven years before that for the next previous sub-1:42 mark. IF Amos trained differently and harnessed his 44-high form to a low-3:30 15 then he would be closest but it's too late for that now.