Well I kinda feel that this buys in the the same principle that it is boring to have the PCs stand still and looking at a Mystery for 10 seconds and then go "aha" and they get it solved after just three-four turns.
I tend to do it this way, and i think it's more or less an official way from SoS but can't be bothered to look now, that for more interesting, and well plot related mysteries, with a bit higher Opacity then say 1-3 you need to start looking into other aspects of the Mystery. Say you bring an Oppacity down to 3 from 5 in a few turns, after that you need to go get another...