Great question. I fished with a guide once who was using Anchovy; dummy flasher on one side, in-line flasher on the other side. Dummy outfitted the in-line 2-0. Very non-scientific study. Most use an in-line flasher with a spoon. I NEVER do. Remember that guides are usually fishing with rookies; as such they need to hook-up lot's of fish, so it makes sense for them to use a in-line flasher. If there are lot's of fish & they are in a biting mood, Iv'e caught large Salmon on a spoon dead in the water.
I doubt that an in-line flasher adds much action with the leader lengths mentioned above, so the flasher is a visual & sonic attractor only - might as well be a dummy then IMO.