The science of finite analysis, structural stiffness and stress is certainly too involved to adequately discuss on this list.
I respectfully add my opinion that a test of bicycle frames, identical in all ways except the top tube is not the best use of financial resources or physical effort.
Certainly, various previous studies have shown the generalized average of stresses as they concern the top tube and although only empirical data would substantiate this completely, in my own studies, actual deflections have closely mirrored that of the finite analysis conclusions.
Would the top tube diameter and thickness add or subtract to the overall stiffness of a bicycle frame...absolutely.
For unit weight would it contribute to the overall stiffness and load as efficiently as the downtube. No.
Theses could be written on this and they have. I personally have been part of three graduate studies of similar items and this has shown the generalized stresses fairly conclusively. Both through finite analysis and empirical deflection testing.
Maybe, an effort like this would be better put to use studying other aspects of frame design as the top tube is just not a major design element in modern bicycles.
I would normally back this argument with empirical data, but it would require many pages of explanation.
Sincerely,
Dave Bohm
Bohemian Bicycles