It's a perfect example of how 'myths' are born.
This is not a myth just hard to prove on a internet forum . I didn't say it was only water cooler talk . My experience of running 9mm and 45acp over a chrono indicated to me regardless of cartridge when I was push Titegroup hard ( max load recommendations ) is when my ES/SD would start to get squirrely ( inconsistent ) .
When I do load development I check weight my scale and weigh out every charge and if we all don't know by now my pistol case prep is close to rifle case prep . Always load same head stamp ESPECIALLY during load development . I have complete confidence that my loads during load development are as consistent as I can get them . This lead me to conclude when you push Titegroup hard it starts to act erratically . I have since talked with many reloaders that have independently came to the same conclusion .
UncleNick does however bring up a good point about powder position . I can't say I was paying much attention to that as he described . I can say during my test I never held the firearm either straight up or down before firing . All my pistol load development is done with my wrist resting on a bag and the gun pretty much stays horizontal through all shots fired .
As far as internet myths , this ain't one of them . Titegroup being a bit erratic at the top end has been around a long time .