I'm sure that you know a hell of a lot more about this than I do, but I've always understood that it's preferable for an air conditioner to run long cycles because it puts less strain on the mechanical elements the fewer times it has to stop and start. In addition, it's uncomfortable for the air coming out of the vent to be way colder than the ambient air.

So while I totally agree with your assessment from a physics point of view, my understanding of the real world application is different.

In addition, I have found a decent amount of confirmation online about the 20° differential thing. This also seems to make sense to me. The evaporator is going to be as cold as it can be, which seems to me is going to be below 60° at the absolute most, but it cannot transfer all that energy from the passing air instantaneously, and most air conditioners (mine, at least) run air past the heat transfer unit at a constant rate, which all leads to a relatively constant level of cooling.

Again, though, I'm sure that my limited understanding is making me look like an idiot.

Am I correct in remembering that you're in Louisiana? Maybe AC units down there are sized differently than they are here. It's hot and humid in North Carolina, but it's no Gulf Coast.