The EEC-IV Extender aka Ford Racing RPM Extender. I don't think they make it anymore but it is a programmer that allows you to control your air/fuel and set your redline. It also gives you the option to eliminate the shift retard at 2500rpms. It's no Tweecer, all you can do is add or subtract fuel. You cannot datalog either.
The extender is in the same class of pcm enhancements as the pro-m A9L retro-fit IMHO.
They work for sure and you will get a gain in power from them. If you're not going too far with your combo and you don't want every last bit of power then they just might be what you are looking for.
The most gain in drivability, power or both will come from a custom tune by you or a pro.