Ah! The age-old conflict between religion and aerodynamics.
Boat tails are relatively safe, except you don't want to cut too close to things on the outside of a corner.
OTOH they look to me like a literal pain in the b*tt. I can't consider one for my XFi because the parking space I have is limited.
The next most effective is the boxed cavity with separation edges. No additional length. Compare an earlier Prius with the later models. They added separation edges on the rear bumper.