This is not correct. A slide made for .40 can be used for .40, 9mm Para, 9x21, .38 Super etc. I have interchangeable breechface on my SVI but I don't change it when I put my 9mm Para barrel in the slide. The guy who owned the gun before me had a .38 Super barrel installed.
So my .40 slide has been used with .40, 9mm para and .38 Super barrel with the same extractor with total reliability.
\\gorgon