rear hatchback top spoilers

my new Toyota GR Yaris has a "spoiler" sitting above the rear hatch glass. it seems most modern hatches have these and my understanding is they try to move the point of air separation further rearward, altering the wake behind the car. there seems to be differences of opinions in the few theoretical papers ive read, as to the benefit of them angling slightly downward following the roof line, versus angled upwards to try to drive any vortices further upward

Ive not seen any factory ones with a up-tuned edge/lip or gurney flap which it seems, in theory, could be helpful.

does factory testing not support their use or perhaps the incremental extra cost cant be justified?

could anyone enlighten me on the whys and wherefores of this item .

im in the process of converting one of these new "hot hatches" to a circuit race car, and thankfully, the category rules give me some freedom to alter things like this, but i dont want to waste time if the idea is a 'dead duck"

thanks