Based on the mods in your sig. Looks like all you have done so far is exhaust.
If you want some cheap performance, take out the snorkle silencer thing that goes from you filter box into the fender. It's free and it'll probably do atleast as much as a TB spacer.
Unless you plan on getting a twin screw or roots supercharger in the near future, you would probably benefit more from an intake spacer (goes between your plenum and intake), you can get one from mustangtuning.com for $36.99 and it's actually slightly thicker than the steeda one.
If you can afford it, Get a short shifter. Mustangtuning sells one for $119. That's an improvement that you'll really appreciate.

 

What is the spacer made of? Penolic spacers add hp by keeping the incoming air temperature lower. I wouldn't spend any money on it if you don't have a CAI.