The mud flaps are there for a reason, they will reduce your "wake" in rainy weather and wet roads. You would be better off boat tailing them... I removed mine (because they were old and brittle) and saw no improvement, I have been thinking about boat tailing the wheels, but haven't gotten around to it yet.
Removing the spoiler should help, replacing it with a Kamm-type-back would help, check out what Weather Spotter has been doing with his vehicle... you both have similar hatch setups, you may be able to leverage off his experience.
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