I have done tests like you described, although not in mono. I have messed around with three Hsu HC-1 mk2s to hear the difference between horizontal vs vertical MTMs. I also played around with that when I had the RSL CG5 and CG25 speakers. I set them up so that the MTM was the center with the MTs as left and rights, and I tried that vs the MT as center and MTMs as left and rights. In my experience, the difference is subtle and I really only ever heard anything (or thought I heard anything) at off-axis angles where cancellation lobes would start to affect the response. I didn't hear the difference on-axis. Even as a pretty strong advocate of three-way centers that eliminate this problem, I tend to agree with you that the problem might be overblown. Although I have to concede that there may have been factors in my experience that isn't true for others, like my room's acoustic properties, or my own biases affecting my perception. I still recommend a three-way center when they can be accommodated because horizontal MTMs can sometimes have a very narrow angle of a good response before the lobes take their toll, and that can degrade the sound for off-axis listeners, but if you are the lone listener sitting dead ahead of the center, I think a horizontal MTM can be just fine, but then again you would be in a spot where a phantom center would be good as well and would scarcely need a physical center.
