IME and research closing the hole you tear in the air neatly behind you is more important than opening it for drag.
Here is a good basic text on Aerodynamics. (same book as the .jpg
)
Aerodynamics of Road Vehicles book review by Nick Demma
It will require gearing and aero to see a big change as the aero will allow taller gearing for the same power.
My experience is HPV's where the power is severley limited so you do whatever you can to get drag down as reduced drag = more speed for the same output.