Hmmmm...thinking about this more, I think the holography is a function of the lateral reflections and relationship between the listening window and sound power curves. Reflections from behind the speaker are mostly due to port output and cabinet diffraction and should be absorbed at the front wall. With that, assuming the listening window and sound power curves of in-walls is "similarly good" to floor-standers/bookshelves, in-walls should be the best in term of soundstage because their rear reflections are absorbed in the wall. The second best would be on-walls. If this is so, the only advantage of floor-standers may be improved placement flexibility and lower bass output.
Note the assumption above for the listening window and sound power curves of in and on-walls being similarly good to floor-standers is a BIG assumption that I have absolutely no basis for.
