I'm wondering which aero modification would be the most beneficial for this hatchback. And no, boat tail is not an option as all vehicle modifications are forbidden in my country and it has to look OEM
On the left is a base model with no aero.
In the middle small trailing edge extension is added as can be seen on almost all modern production vehicles nowadays.
On the right spoiler is added with small gurney flap on the top to make angle between top of the roof and top of the spoiler as shallow as possible.
This design was quite frequently used in the 80's (Kadett GSI, Audi 100 wagon, Car and Driver Pinto) and is proven to work but nobody uses it anymore.
Is it because trailing edge extensions are better at lowering drag, or is it in fact superior solution and manufacturers abandoned it only because it might not look "cool" enough for today's car buyers?