I’ve run 15 marathons with 6 of them at Boston.
PR - CIM 2009 - 2:53
Boston - Ran 2:55 twice, one of them was the course record year with the tailwind, so after my PR. One was right at 3 hours. One was 2011 which was hot, ran 3:11 (1:28/1:43). The other two were my first and last times around 3:14-3:15, not in great shape for either of those.
Since I can’t really train for the hills where I live, I think Boston is about a 4 minute harder course for me than something flat. Even the tailwind year probably just brought me back to even.