Throttle body spacer- performance gains?

No, it really doesn't help. Some claim it reduces the heat, but if you think about the heat would go inward not out because the heat in the engine compartment is alot hotter than the heat inside the tube coming from the fenderwell. Plus the air moves too fast. Some of them create a spin to the air that only create turbulance when being pulled downward into the engine, the only way to atomize the fuel/air mix better is inside the chamber not just after the throttle body, and the spin has to be rotational inside the chamber hence why some companies use dished pistons that are engineered a very precice manner to create the spin. It does make a good, overly large paperweight though.

Without other supporting modifications requiring a larger amount of incoming air, it'll do absolutely nothing for you.

On a V6 Mustang...I see no benefit nor have I heard of any. The SOTP meter is normally someone rationalizing the money that they actually wasted on the product. Heh, no one like to think they bought snake oil.

Now phelonic spacers in certain applications do work....it will keep the heat soak down...which might have a small gain from that.