Here's an overlay of the sedan vs coupe. The rear window is indeed steeper, and the trunk is a different shape. Both of those mean we would want a longer spoiler.
Watching the morning dew evaporate, I conclude that my sedan has detached airflow from about halfway down the rear window. The bubble makes an arc from the lower corners up to the center of the window and back down.
(I don't know what's up with the front wheel. I took both images from Tirerack's "buy wheels" preview and photoshopped to get the outlines. I matched the rest of the body and had to ignore the front wheel)
