A rear spoiler at the end of the Prius could have been put there for several reasons, and unless we can talk to the aerodynamicist, we'll never know for sure which (or which combination of these) applies:
- as a styling element (ie. aerodynamically useless or even detrimental)
- to reduce rear lift
- to reduce drag by changing the "effective" rear slope
- to reduce drag by forcing cleaner separation where airflow might otherwise "wrap" around a surface
I would just carry on with what you're doing and not worry about a spoiler. The angle you're working with is fine, and that's the most important issue.
FYI, in your previous post you said you weren't sure if you learned anything from the Ranger tuft testing video. The key thing there was we saw the tufts streaming aft on both the sides and top surfaces of his aero cap, which indicates attached flow. It confirms that his angles weren't too extreme and that he should see an efficiency improvement from his work.