MazdaMatt: probably useless, yes. You're going to have detached, turbulent flow coming off the air dam.
Raysp30: shaped deflectors like the kind the Autospeed author tried will be better than the flat "spat" type OEM's typically use. (But more likely to be damaged on stuff inside approach angles, and harder to build).
Aero concept cars usually go with shaped deflectors, eg:
