They don't look large enough to do much. Autospeed tested some (first four links here) and Mitsubishi found that "Application of the VGs of the optimum shape showed a 0.006 reduction in both the drag coefficient and lift coefficient of the Mitsubishi Lancer Evolution." There are also some VG's that cost about $6, a huge saving over these ones, with the benefit of enabling experimentation to see where they are most effective.