It's cowboy loads that often fail to seal the chamber in a .45.
You are correct, higher pressure loads don't generally have that issue. But we are looking at the comparison to .44-40 not the .44 mag.
As far as using .44-40 brass in the .45. ??? There are more dimensional differences than the neck diameter. Dies won't change the base or rim diameter. As thin as those necks are, I wouldn't care to stretch them.
