[svxistentialist]

I think it is probable that the rear wings or spoilers, either the US or JDM style, were put on for style rather than for function.

In addition, it is absolutely CERTAIN that the cars with the rear spoilers have a far higher Cd than ones without.

In which case, there is a trade off between what beneficial rear downforce is created by the wing, and at what constant speed this force is most useful, versus what negative drag effect the wing creates at normal road speeds of 80 to 120.

Personally, I prefer my car slippery for better penetration. When wet or when dry.