[1QuickS]

It is easy to demonstrate the torsional stiffness between coupe and Targas:

Get a toilet paper tube and twist it. Then, smash it flat and try twisting it. The amount of material is the same but the torsional stiffness is dramatically reduced. The flattened tube is what a Targa is like compared to a coupe so adding any structural connection across the opening will help chassis stiffness.

Basic torsional stiffness is a 4th power function of distance from axis of twist "neutral axis" to each element of the structure. If the structure is a coupe then there is a greater distance from the neutral axis to the shell of the body vs. a Targa that has very little distance in the vertical direction. Coupe & Targa have the same width so they are comparable in that area for contribution to torsional stiffness. If the tunnel has increased material thickness and the sills also have thicker material then the Targa will have more contribution to torsional stiffness contribution in those areas than a coupe.

Front to rear bending of the chassis is similar in stiffness comparisons. Try bending the toilet paper tube before and after smashing it flat.

Since the Targa has no load path across the open top to transfer load other than the pan of the chassis it follows that torsional and bending flexure (stiffness) pale in comparison to that of a coupe.