I have talked to two different HVAC companies and our system is sized correctly. When I complain that when it's 105 out I'm lucky if it can hold 80 they tell me it's working as expected. They say no system can do more than about a 25F drop but I have a hard time believing that. People in hotter climates somehow are able to cool their homes more than 25 degrees and commercial spaces (like stores and offices) somehow manage to cool more than that.
On the other hand, I'm asking them to sell me something that's more expensive and the fact that they don't want to tells me that they have some reason to believe I wouldn't be happy with the result.
It costs money to do a schedule J measurements so it's unlikely you've got it from just talking. What you describe sounds like an undersized system. Max 25 drop is obviously false, I have solid 75 with 115 outside with a residential AC, for example.
The explanation in the article does not make sense to me unless it's talking about window units. Ducted systems do not cool the outside air so the amount of temperature drop in the air passing through is not relevant. As long as it can cool faster than the air is being heated inside it can drop the inside temperature to the temperature of the coil, which is unlikely to be set at 80 degree in your case.
I can't find a single article saying anything else. The 20-25 degree maximum temperature drop seems to be everywhere which leads me to think it must be true but I don't understand why. I'm trying to find out what happens if you go oversized on a variable speed system and can't find any information.
On the other hand, I'm asking them to sell me something that's more expensive and the fact that they don't want to tells me that they have some reason to believe I wouldn't be happy with the result.