First of all, we are limited to 3D models, and not every car has an upgraded version. We all know that Zoqqer's Aventador SV gives you bugs, like the driver being away from the car, going through the window or anything like that. So, i said this before and i will repeat. I avoid using Zoqqer's mods because of such bug. Specially his Aventadors.
Second, TDU's F40 is not a 1980's model, it is a 90's built F40, which removes it from the G class category, different than the 959, which is older than 1989. Any car, whatever is the performance, older than 1989 is categorized as a G class. TDU had the Corvette C1 in same category as a Lamborghini Miura. G class is "classic" car class. And not "slower than F".
Some cars have a lower category, like some mercedeses, because their weight and cornering hability. Power and speed do count, but also does cornering. And those big sedans often can't take them.
I'd suggest, instead of just look at the stats, you guys create a good mixed track, with corners and straights, and actually test the cars. See the lap time you get, and then i believe, the classes will start to make a bit more sense.
Edit: Rulezz 3D models are incredible wrong, and i also avoid to use them as much as i can.