Here's my 2¢.
You shred the air at the top of your rear window.
If you really want to improve the shape, you must follow the template with something solid, a roof spoiler or a rear deck lid extension won't do much.
The air needs to "ride down" on something in order to return its energy back to the car, by the time it hits the extension it is too late, if you cut off flow with a roof spoiler, you really aren't improving on what you already have. The current shape is not terrible, it just isn't optimal. You have to add a "Kamm" extension to optimize.