From everything I've seen posted I think your right but since it was offered and I'm going to have the rear appart to put in SS Lines, new Disc's & Pads I might as well prove to myself that the 2-pot won't fit a 3rd gen Legacy without a new backing plate. Not a big deal.
I also keep wondering if there is a way to do an adapter bracket.... gotta get some measurments.....
I did go back and check and it was you who posted a picture of a backing plate with two sets of holes in it in a thread in the suspension forum. Do you know where that was used and is there a Subaru PN for it ?