Peter,
That's true in general what you say about the imaging but I suspect that in a room that large, if it also has a high or vaulted ceiling, all talk of timing and "imaging" go out the window because of the huge reverberant space. In one such room with a cathedral ceiling that housed an excellent pair of large Canadian floorstanding speakers plus two big subwoofers at one end and the couch at the other, I did some listening comparisons with several of my listening panel members from the National Research Council.
What was amazing was that when we switched from stereo to mono, none of us could hear any difference! It was simply a huge reverberant wall of sound.
In a related experiment conducted in more formal testing conditions (double blind) in the NRC listening room, we discovered that simply running a second set of left/right main speakers set a few feet farther apart to either side of the main pair of L/R fronts produced an amazing improvement in overall realism and spaciousness, something akin to what we now experience when we listen to a stereo recording in DPLII that decodes really well into 5.1. At the time, were testing very early digital and analog "bucket-brigade" delay systems that routed the main channel signals, delayed by an adjustable amount, to several surround speakers on the side walls. This was long before the advent of Dolby Digital or Dolby Surround.
The addition of the extra stereo pair of main speakers up front provided as much realism as the delay devices, which were quite expensive back in the early 1980s.
In a room that large, I'd say go ahead and try an extra pair of M80s or M60s set up as described. Depending on the room, it might deliver a noticeable improvement in spaciousness and realism.
The NRC test was done in a standard IEC listening room that parallels a typical domestic rectangular living room of average dimensions.
