I got a bit of work done in the measuring/comparing department.
I found a better profile image of a 5-door Aspire, and matched up the template as best as I could (even estimating my ride height as compared to the image's):
With the spoiler, I was looking at it's height and how close it would bring me to the template:
Estimations put it at just around halfway between the bottom of the rear glass and the template. I also laid the profile shot over the Prius overlay, matching the templates up perfectly.
You can see it's not going to take a lot of additional height to bring the angle up to Prius territory. I'd like to find some wind tunnel images/video of the Pruis to see if I should go closer to the template, or if a Prius profile is close enough.