Most of us have stereos in our houses with the speakers no further apart than that (4ft away from center - 8ft total)... Often they are closer than that.

I guess we can all go and sell one of the speakers, if we can't hear any difference beyond ten feet.

Things like this are easy to test. Make two CD tracks from a CD rip. Leave one stereo, and make the other mono, and panned to one side. Play them both back to back, and get over ten feet from your stereo. Notice a difference?