I want to say this cautiously. I am no expert by any means but I am 99.99% certain that the vortex generators are not used to reduce drag on the Lancer. Rather they are used to clean up the airflow into a more laminar flow over the rear spoiler to increase down force.
The IIRC is a small 4 door, 4 cylinder sedan with a huge turbo and the car is designed to go like stink and handle. Look carefully and you can clearly see the vortex generators several times in this movie.
Trust me, they are for downforce only, you can not even mention fuel economy and the Lancer IIRC in the same sentence with out being laughed at. There are only a very few cars that are faster or handle better than this car and they all cost 3 times as much.