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If the rear bumper cover was acting as a drag chute, and you cut holes in said drag chute, you'd probably reduce drag. So, "speed holes" may be a perverse way to partially correct poor design.
Better not to have a drag chute, but using an undertray or such to smoothe the airfow.
And, start with a decent plastic garden edging air dam in the front, to keep the draggy air from clogging up the underside of the car in the first place.
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