"Guns tend to work best when chambered for the calibers they were originally designed for."
I agree absolutely.
In other words, by this logic, we'd say that a 9mm 1911 would not work as reliably as a 1911 chambered in .45 ACP. What do you guys think of this statement?
I think you are making a leap to a conclusion not made in the original quote.
"Tend to work best" is not saying something else will NOT work, or not work well, only that the overall, the original design works better than a modification.
Your statement about 9mm vs .45 1911 is making a couple of assumptions, not supported by the original quoted passage.
First, you are assuming that a 1911 in 9mm is not a round the 1911 is designed for. If it comes from the factory in 9mm, one can make the argument that it is "designed" to handle the 9mm properly. IT may be a refinement or modification of the inventor's original design, but rest assured that factory engineers have done the work to make it operate properly with the 9mm.
And, second, you are assuming (for the sake of argument) that 9mm 1911s do not work as well as .45 cal 1911s. As far as I know, that's not the case.
As for the Rem XP-100 pistol, I wondered for years why they came out with the .221 Rem Fireball cartridge, when the gun will easily manage .222 Rem, or .223 Rem. Finally I figured it out. That gun has a 10" (or maybe 10.5"?) barrel. You can certainly chamber it in a larger cased round, but the .221 case holds all the powder you can burn in that barrel length. Using a .222 or .223 case just means that you are "wasting" the extra powder for no real gain.
Plus, of course, it meant you had to buy a different Remington product (the ammo), so, more profit for Rem!
Lots of gun modifications work just fine. Some don't. I think the general statement about "tending to work better" is true, when you look at the overall numbers of individual guns involved.
Plus, my sig line uses the same general principle,