It's all speed dependent. I found these graphs from a Cummins white paper. It's geared towards semi trucks, but it illustrates the principle.
Here's the first plot. It shows where the power goes vs vehicle speed:
The red portion is the power required to overcome the aero drag. Obviously, the % benefit you're going to see is dependent on the vehicle speed.
This second plot (from the same white paper) shows % mpg benefit vs % aero improvement for different duty cycles:
Note that a low speed duty cycle (inter-city) shows very limited benefit from aero aids vs a high speed duty cycle (interstate) show substantial mpg benefits.