Yeah, they all have decent downforce, they just need to be tuned appropriately. I'd say any of them could outperform a Gr4 car handily.
I just checked and you're right. The rear downforce on my 86 GrB maxes at 200 while Gr4 cars have 260. I'm pretty sure their GTS numbers were higher than that which is what I was assuming.The downforce on all of them is awful as far as I have seen.