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Many efficient cars that have "spoilers" really just have deck lid lips/extensions to encourage flow off the trunk to separate cleanly, rather than curving partway down the rear of the car before separating (which increases both lift and drag).
So here's my GUESS: IF your trunk (without a spoiler) has attached flow at its end (and I'm guessing it does), and also a relatively sharp corner in the metalwork transition to the back of the car (no idea), then that raised spoiler probably hurts since it would increase the size of the trailing wake.
But that's only a guess. And unfortunately, I bet the difference would be small enough to be hard to detect in coastdown testing. But you could always try. That would give you a definitive answer.
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