as Duncan said - 120km/h on steepest setting is 4kg. That's nothing. Aero devices really have minimal effect until your speeds are higher (think 180+).
Removing a rear lip on your car is a slightly different story, as the lip tunes the airflow leaving the back of the car. I can certainly see how it would make a difference at freeway speeds, if the change is significant enough. For less significant changes you'd probably again feel it at 180+, as URAS found.