Install or not install?

You'll probably take a mileage hit...

 

Doesn't look half bad. The owner in the pic definitely takes care of the car, so make sure you do it right. A half a$$ed installation will look really shoddy.

 

Just my honest opinion, nothing personal, I think it looks very rice. So far, your other mods have been very clean and well done. I think a mod like this would make it look like those riced out civics we see around LA and Inland Empire. It looks cheap and not right.

 

I would vote no on this one. Your car is so clean and elegant looking, this would ruin that look IMO.

 

I wouldn't do it. It will be more a drag in aerodynamics than a push for downforce.
Rear deck wings become active in downforce after 110 -120mph for most cars.
But it's your car

 

NOOOOOOO! You have one of the best looking Prius out there. Don't ruin it. What's with the exhaust? I missed that add. Tell me more. Just say NO! (My humble opinion)