I've never had a Legacy so can only go by what I see and know about from the Impreza.
And I've been telling people for week's that have been selling there Impreza's not to do it :D And now I seem to be right ;)(Feel's Smug)
I had an idle problem with another car no engine light at all, turned out it was the MAFS :( To be fair I even bit the bullet with that car paid someone to check it with smoke and diagnostic came back fine could not find a thing, really made me mad that :D So I found myself like you reading up and changed it and it worked. But that was when the clutch was down and slowing to say aroundabout, so dout it will be that. I would have thought a coilpack or plug would have given a misfire that you might pick up on, so worth a look maybe.
And I would not know how to check the Injector but have you changed the fuel filter just on the off chance ?
And maybe try some good strong injector cleaner. Don't think it would do any harm.
And thank's for keeping us all updated appreciated :D