Problem with the examples given in this thread is that most, if not all, of the improvements are due to not using warm under hood air and are not due to ram air effect. NASA did some testing and it really isn't until you hit 100 mph that any noticeable effect is seen. Of course design (getting above the boundary layer)is important and I don't see any of the pictured scoops designed to do that. I suspect that the mixture issues mentioned are due more to turbulence of the airflow through the carb than a pressure increase in general, after all that is what makes a carb work.

For a street cars, it's for looks. A race car may see ram air effects at the top end of the run.