What, IMO, does make a difference - directivity. Most High efficiency loudspeakers are more directional because they tend to be bigger. This directionality helps to minimize room refections and diffractions which are audibly very negative factors. But high directivity in and of itself is not enough, it must also be well controlled. Both the direct response and the power response have to be equal and this must be done with a high degree of directionality to achieve good imaging and presnce. but the room itself must also be done right or the loudspeaker won't make a whole lot of difference.

No, I don't think that its the high efficiency of larger drivers and waveguieds that matter, but it is there inherent narrower directivity that is the primary factor.