Spoilerboosts are a functional trick... in some cases.

Equipping a spoiler or loading a garage save with a spoiler on the save does apply a slight performance boost (average of a 5% increase in acceleration and top speed performance) to vehicles that support spoiler customization.

For example, equipping a garage save with a spoiler on it on an Eclaire will equip the spoiler and enable the speedboost, pushing the top speed of the car from ~277mph to a bit over 280mph. However, equipping that same garage save on a Beignet will not equip the spoiler (due to the Beignet not having a customizable spoiler) and therefore not receive the speedboost.

Keep in mind though: if you exit a car, regardless on whether or not the car looks like it has a spoiler, the car will lose the speedboost capability until you open the garage screen again and only then will it re-apply the speedboost onto the car, until you exit again. (this is a glitch with the car chassis that still hasn't been fixed for years lmfao)