whereas 24 fps slowed to 40%, even using AE, doesn't look nearly as good.
You could be right. I personally think 40% slow-mo is too slow. I feel 60% is the perfect rate for slow motion footage, whatever the original frame rate. Another reason I don't advocate 60p. It naturally goes to 40% in a 24p sequence, and that's just too slow.
However, 24p slowed down to 60% in AE looks better, to my eye, than 60p reduced to 40% in a 24p sequence. Combine that superior 60% speed with the superior normal playback of 24p, and we're back again to shooting 24p as the norm.