nessarily the culpurit. If all things were the same, glass is stiffer than Japanese sheet metal. But where I would agree with you is that two rails above the windows are cut and basiclly missing, and replaced with a single support in the middle. One support in middle vs two on the side would tend to be less stiff.
Lastly, does it even matter. The T-tops are a part of the body. Doesn't the frame rails determine the chasis stiffness?