Group delay and regular propigation/time of flight delay are two different things.
Put simply-Group delay is the rate of change of of the phase.
Time of flight delay is the difference in the time the signal is produced-until it reaches a particular point in space.
With the Tapped horn-the "time of flight" is not the total path length. Nor is it the path length from the "other side" (direct output side) of the drivers.
It is "somewhere in between". And it changes with freq.
There is more to just "accounting for the delay". You also have to consider the phase response with the mains-around crossover. If you get one of those dead on-but the other is off-you could have a huge hole in the response.
On the other hand-if each one is a little bit off-you have a have a decent response through crossover.
And once you start to "account for" the time of flight-that would assume that the subs and the mains are in the same location. If they aren't, then as you move to different seating locations-the time difference between the two is going to change.
Not trying to be picky-but there are often many more things to consider when "setting up a system"-rather than just a single point-or single location-unless that is all that is important to you. And sometimes a single point is the most important-other times not.