Honestly I'm kinda sad about the rating I have to give to this car, because it looks great, it handles and sprints fantastically being so light and it's just great in corners... perhaps too great.
Downforce figures are seems above those of cars from the same era, according to the Wings App this car has 700kg at 300km/h, which sounds kinda for a '73 car that doesn't even have a diffuser.
To give a context, in game that's the same downforce of the much bigger Porsche 917/30, while according to some data found on
http://www.mulsannescorner.com/data.html cars of similar age and shape like the 936 don't even reach 400kg at 300km/h.
Wrapping up, it would be a 5/5 for fun, but this is supposed to be a simulator :(
I have been re-making physics on the "C" version and the aero is closer to what you describe. Future update will eliminate the non-C version completely.