Autospeed has quite a few aero articles related to this sort of discussion
Vortex Generator 4 part series (They specifically test the vortex generators on the underside too):
Vortex 1
Vortex 2
Vortex 3
Vortex 4
Also in that article they show some data from Mistu EVO tests that seems to suggest that 25mm high or so is best.
Aero Testing 5 part series:
Aero 1
Aero 2
Aero 3
Aero 4
Aero 5
Underflow 2 Part Series:
Under car 1
Under car 2