That spoiler on the Prius doesn't extend very far. Probably for aesthetic reasons? My idea behind this is that airflow reattaching is connected further before detaching behind the car, thus reducing the rear area
similar to a bottail but obviously far less beneficial.
After I get my O2 sensor thing fixed I'm planning on tuft testing it and experiementing with VGs. If I can get airflow to remain fully attached down the rear window, that along with the tain extension should mimic at least a partial boattail I'm thinking.
There's actually several similar examples on this site, albeit from quite a long time ago. It's more of an enlarged version of what 19bonestock88 did on his Ion, and also similar to the
rear deck extension endplates that was tested on a same gen Civic coupe for a 1.2-1.4 mpg increase at 60-70 MPH.