@Glockadopoulos great write up.
I agree, and apologize for not mentioning the 9mm cannot be converted to .40, and that a factory 9mm barrel will not work. Perhaps you already knew this, but I should have remembered to mention it, at least. The conversion barrels for 9mm will have the same outside diameter as the .40, and will fit just like a .40 into the slide, but the bore will be 9mm, resulting in a thicker walled barrel.
I've only tried the Lone Wolf conversion barrels and they work. Very nice machining and accurate.