Clarification:
There is no difference in the feed ramp as far as I can tell, my gun will feed fine as long as I use 9mm mags to feed 9mm ammo from(my .40 mags didn't feed 9mm worth sour apples, but that's not what they were designed to do).
The difference is in the 9mm extractor getting a better bite on the 9mm case rim than the gun's original .40 extractor would, and in the 9mm ejector being angled inward to properly impinge on the 9mm case head as opposed to the .40 ejector, which is not angled to avoid the possibility of the ejector impinging on a live primer instead of the case head when clearing the chamber
(during the R&D of the G22/23, Glock found that this could happen with the then-standard G17/19 ejectors, hence the different .40 ejectors).
Basically, a .40S&W gun with the correct extractor and ejector for the original caliber MAY have a SLIGHT-but-undesirable chance of failing to extract or eject properly when the gun is fired with 9mm using a conversion barrel - hence the injunction to not use the gun in that configuration for self-defense. Installing the appropriate 9mm parts reduces that slight chance even further.
I don't see the .40 slide's wider breechface being a problem, because each 9mm round will self-center on the breechface as it is chambered(assuming that the conversion barrel is manufactured correctly, and the chamber is not off-center with respect to the firing-pin hole in the breechface).
That said, I still use the 9mm-converted gun only for practice and competition - not because of reliability concerns
per se, but because I prefer .40S&W as a defensive caliber over 9mm. However, if circumstances were such that I were completely deprived of the correct .40S&W ammunition for the gun, I would convert it so that available 9mm ammunition could be used
in extremis as a last resort for defensive purposes.