Try doing a search since this question has been asked many times.
Throttle body spacers have some effect on carbs and TBI systems but not on SEFI. Most consider the spacers useless but they won't harm the engine. Some spacers do have a whistle sound at certain RPM.
I don't think they add any power or increase gas mileage but thats only my opinion, and like belly buttons everybody has one.
YMMV
rikard