Here are a few classic "official" PR's from the sub 3.30 club...
Caleb Ndiku 1.52.2
Narve Nordas 1.58.63
Stewart McSweyn 2.01.48
Clearly those "PR's" don't mean anything but the one guy who I would bet my money on being the slowest is Lamecha Girma. He's not far under 3.30 at 3.29.51 and when he ran that he had a pretty clean race able to key off both J.I and Kerr so I don't know if there is too much more there. But his endurance and threshold are clearly incredible - especially as a sub 7.24 3000m guy and a sub 13min 5000m guy.
My guess is that if Girma could run much faster than 1.48.5 I would be surprised. The other guy I thought long and hard about was McSweyn and could he run under that but I think he might be able to. He has got to the same time over 1500m as Girma and is a similar runner in that he has excellent 3000m ability too at 7.28.02. But he paced that Silesia race and was just under 1.51 at 800m and then was able to break 56 for that third lap so I have to think he is a 1.48 flat guy give or take a few tenths. I think him and Nordas are probably at a similar ability over 2 laps.
Can someone break 3.30 with a 1.49 800m PR? To break it you have to be capable of running at least 1.52.5 through 800m. I mean I personally find it unlikely - a 3-3.5 second differential between 800m ceiling and interim split seems really tight no matter how strong over 3000/5000m you are.
As for Nico Young? If he gets a start in the right field this season he can break 3.30. I see this happening before a sub 1.47 for him.