The subframe isn’t a brace…. As intended. But if it’s solidly mounted on metal bushings is effectively a solidly part of the unibody structure, whether it benefits it substantially or not. I will firmly say though given its steel is literally twice the thickness of the unibody stampings it’s probably doing more than adding a few twigs between the LCA mounts.
There’s still the question of what I asked though; what problem would it solve? Beyond the challenge of maintaining composure from engine torque the IRS is far less critical when it comes to maintaining geometry than the front suspension and aa far as I’m concerned if you’ve got stiff bushings, good shocks and springs and an Addco back there any piece of brace you’re adding back there is ballast at best.
That’s not true, well designed subframe connectors undoubtedly work, however I find *bolt in* connectors(which given these cars fuel tank are inherently stuck with) inherently compromised and a net zero or negative in vehicle performance for the weight and irritation they add.