With a finite rather than infinite segmented wall, there will be left and right boundaries to the listening area(shown in the figure) you would need to stay inside of for the image to stay centered in front of you. The listening area boundaries narrow as delay is increased and the angle of the virtual sources are spread further apart. Also, moving further away from the wall of ESLs narrows the left and right boundaries of the listening area.
But as you move off to the left, isn't L getting louder and earlier? Isn't the delay (and distances) making things exactly worse? (Isn't that what "toe-in" of speaker boxes tries to over-come?)
Ummm.... just occurred to me: didn't Walker feed from the centre on the Quads (for frequency and beaming if not for spatial image reasons)? That would change the whole analysis.