Been there tried that...Only because I got it for free. It made zero difference in both gas mileage and performance. Get a tuner, or a free flowing exhaust. The only benefit is that I took the time to clean the throttle body while I was there. Save your money.
Aside from everyone saying they are worthless, I'll give you some reasons. A throttle body spacer was originally designed to help mix the air and fuel in a carbeurated engine. In our injected engines they have no real use. One common argument and a common selling feature is that they 'swirl' the air before it enters the intake manifold, well, that also slows down the air velocity by creating a turbulent air mass in the throttlebody's constriction point. Ideally, our keg-style intakes need higher velocity into them to make their best power, so slowing this down with a tbs makes no sense. You are basically looking at a piece of equipment that is a hold over from old technology, but because it's been around fire so long everybody thinks it's a good upgrade.
The only good tbs is one that is tapped for nitrous.