FWIW, you can have your .357 cylinder machined to take .38 super and the rimmed special cases are, as advertised, usable without the moonclips. So the difference in neck and base diameter between the .38 super -> .38 special -> .380 ACP is about even at roughly ~0.006" each step. The difference between shooting a .38 special in a chamber machined to .38 super isn't much worse than shooting a .380 ACP in a factory .38/.357 chamber.
I wonder if the .38 super using .38 special has a similar drop off in velocity. I would assume so.