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.
Edit: I haven't really started my benchmarking process because I don't own enough cars to mess with it but I ran the 86 GrB on my test track and Gr4 suspension/downforce/power/PP and it seems pretty good to me. A bit too grippy up front at high speed, you can't actually sustain full steering lock. Really curious to see how it compares to an AWD Gr4 but without a power boost I have a feeling it may get beaten.