I'm no silencer expert. You just learned about everything I know on the subject.
No question that it should theoretically be quieter with longer barrels. How much it matters per inch, I have no idea. How much it wears on a silencer to have 15,000psi muzzle pressure (which is a reasonable number at 16") dumped in it rather than 9,000 (which is reasonable at 24"), I don't know.