OP. I LOVED this thread. So much so that I spent about half the day yesterday doing the research. I just published the article for our SC members.
https://twitter.com/letsrundotcom/status/1352667505052037123
In the article, I decided to go year-by-year and see if I thought Willis would have won the outdoor US 1500 title in a given year, starting in 2003 when Willis first dipped under 3:40 for 1500. But before I looked at their yearly results, I started by looking at the head-to-head career standings between Willis and the leading US guys at 1500/mile.
Nick Willis vs Leo Manzano
Nick Willis vs Lopez Lomong
Nick Willis vs Robby Andrews
Nick Willis vs. Alan Webb
Nick Willis vs. Matthew Centrowitz
Nick Willis vs Bernard Lagat
Then I looked at how he was doing each year and compared it to the US studs. The yearly results were interesting. There is only one year over the last 18 seasons where I'm 100% certain Willis would have won the US 1500 title. However, there are seven other years where he would have had a decent chance of doing it. And of those seven years, he would have been the betting favorite in five or six of them if we retroactively gave out odds for the US champs after the fact based on their seasonal results.