If there's a curved trunk lip, I suspect it might be helping, but if the cabin shape is non-optimal, it might just be sitting in turbulent wake, or if the cabin shape was intended originally to form a vortex cushion over the trunk before some marketroid restyler stuck the spoiler on, then it might be messing that up.
A relatively quick way to determine whether it was helping or not would be to time some coastdown tests from 65-50mph, if it coasts longer with it on it was helping.
I remember The RoadWarrior..To understand who he was, you have to go back to another time..the world was powered by the black fuel & the desert sprouted great cities..Gone now, swept away..two mighty warrior tribes went to war & touched off a blaze which engulfed them all. Without fuel, they were nothing..thundering machines sputtered & stopped..Only those mobile enough to scavenge, brutal enough to pillage would survive. The gangs took over the highways, ready to wage war for a tank of juice
what you might try is look at the car from the side and try to see where the air would separate at the roofline /rear window, it's around an 11? angle. (or you could use a long ruler perhaps to visualize the line) if you follow that line down and it would end up behind the trunk without the spoiler but with it hit the spoiler, that it could be doing some good, if that line falls before the spoiler if might hurt, if the line fals beside it it won't matter. i'm basing this "opinion" on observations of a lot of cars, it's not written as a rule anywhere.
but the 11? separation point is based on facts and can make a lot of car shapes make sense!
also do the same for the airflow coming of the C pillars as that wants to go to the inside along the same angle...
looks like it could help from the photo.
If not, maybe vortex generators (see stickt on aero) on top of the roof would help reattach the airflow, eliminate the rear window bubble, and allow the flow to roll on the window, the trunk, the spoiler, and cleanly out...