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The fins would help guide the flow air more over the roof, which would provide better efficiency over the trunk wing (which this car doesn't have, so real benefit in this case).
I've seen some fins on the top of some airplane wings to more effectively guide more air over the aileron.
When it comes to aero, the benefits of "downforce" on a car don't have much value under 100+ mph and you will not go faster with downforce unless you are benefitting from the traction gained in a high speed turn. Having downforce to go in a straight line is usless, unless you are drag racing in a top fuel car, etc.
Now aero mods to benefit gas mileage are not about downforce, so the benefits at freeway speeds are effective and could make you "go faster".