I'm certainly willing to believe that with some signal modifications you can cut the time for a follower to arrive at the station considerably. I only measured what's happening now, of course.
But you need to assume SOME variability in the times.
Whatever the minimum resulting time is, it's not realistic to assume that every T/O exactly achieves the minimum. My belief is that 5 of the 7 T/Os I observed tried their best to achieve the minimum, and the results varied from 80 to 87 seconds with a mean of 84. And what do you do about the T/Os who don't try very hard because they worry too much about going into BIE? Discipline them because they took 102 or 107 seconds instead of 80 to 87?
Maybe NYCT needs to negotiate with the union to put in place a rule that only aggressive, particularly skillful T/Os are allowed to pick the southbound Lex in the most critical 90 minutes of the day.
I also think there will be more variability in dwell time than you project. My sample was small, but in 3 out of 10 cases dwell time was seriously increased by events that have nothing to do with crowd levels, and therefore not susceptible to mathematical modeling. Short of making holding a door, or getting stuck in a closing door, a serious criminal offense, I don't think this sizeable variability is going away.