OK you said "Group size at 25 yards with any of my[your] .45's is identical regardless of the cartridge OAL." That would have been good enough. I get most of my groups from the range, and most of my sermons from Baptist preachers.
I find that 0.010" change in seating depth with 5.0gr Win.231 and a Berry's 230RN in a Kimber Stainless Target at 18 feet from a sandbag puts 10 rounds in 7/10". Any seating depth other than that specific COAL value throws them all over a 12x12 paper. I wonder if anyone ELSE sees this type behavior.
And the thing is, when I ask a ballistician they say it shouldn't make a difference. OK! And the guys that got me into the ACP shooting can ding a quarter at 25 yards and they say they've never even thought about seating depth, as long as they get proper feed and function!