You could go lower profile than that even if that's the style bracing you are going to do, and that's all it's going to be is chassis bracing. IMHO if you are going that far, you might as well do a cage. It wouldn't be that much more work and you can put swing-out's in it to help ease entry.
The under-car bracing HAD to do something. Look at Mustangs and the like...they ALL have subframe connectors available for them and they ALL see an appreciable increase in unibody rigidity. It might not be as much as a superstructure style bracing, but it is still enough to make a large difference.
As far as the re-design of the J-body convertible structure, I believe that happened in '90. They basically copied the TC's design where the rocker panels were reinforced to more effectively take on the torsional load of the chassis. Because the strength was increased farther away from the moment of inertia, the unibody now acted more like an "I"-beam and became more resistant to both twisting and longitudinal flex.