I don't know if Second Moment of Area has much to do with tension. It is basially a value of the amount of material and how far it is from a central axis.
It is very important in bending, torsion and buckling, but not in pure tension. Hookes Law does't account for Second Moment of Area.
This is my point. Basically a rim lateral load involves a deflection of the rim, thus a bending of the spokes near this load. The thickness of the spokes is what directly resists to this bending. The width of the spoke also enters into the equation, but with a less important factor.
That is why I'm sure the round 1.8mm DT Competition is laterally stiffer than the new DT Aero Comp 2.3/1.2.
An easy application: take an Aerolite spoke and a Revolution spoke in hand. Bend the Aerolite along its thin section. Now bend a Revolution (1.5mm round). You will see that it resists more to bending.
When the wheel is under tension, the spokes exactly behaves in the same manner in my opinion. Why wouldn't they?
Anyway, I get your and Ergott's points to say that the tensile strength of the spoke is the same whatever the shape of the spoke. In this case, the amount of material is what matters.
Truth is, if the DT Comps are in fact laterally more stable I would go with those (for me, at 195lbs) and say phooey to any aero benefit that might be obtained by flattening them a bit. I'll look forward to your guys' ultimate conclusions on this one.
Another important point is the twisting of the spoke while tensionning it. A DT Aero Comp is flatten so it can be hold well during tensionning.
A DT Competition is round so it can't be hold against twisting. You need a good wheelbuilder for reaching high spokes tensions with a round spoke.
Although round spokes were very common in the past for custom wheels, the new options offered with bladed spokes and how easy they are to build into a durable wheel, have made them a premium choice for most wheels.
Could you elaborate on what you mean by second moment of inertia?
I'd love to know more about your test when you do it. What order of magnitude do you think the differences in stiffness be? How will you rule out other factors?
Please keep me posted to what you find out. I really appreciate the work you do.
Second moment of inertia: above in my post.
For the test, I will take a rim, a hub, and DT Competition spokes. I will build properly the wheel with a given lacing pattern (certainly radial) until it has good and measured spokes tension. I'll disassemble it, then build it again with the new Aero Comp, same pattern, same spoke tension. The rim will be exactly the same, and the lateral load will be applied at the very same spot on the rim.