It isn't downforce as in an upside down wing, it is a spoiler, it spoils the airflow and can affect balance in the air. There is very little 'aerodynamics' involved, what your trying to achieve is a change of 'balance' rather than a massive change in loading to the front or rear axle. Correctly designed, it should aide handling at med-high speed that is all.
I would agree that there is a 'placebo' affect going on, like when you buy a new car, you feel faster, so you actually go faster, it isn't the car it is you.
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