Not some piddling 15 hp either, really substantial gains.
Car engines have always been manufactured with a certain balance between power and longevity. This was generally built into the mechanics of the engine, but recently there has been a lot of power potential that can be found in the engine computer, if you have the means to unlock it. Such a tool is the Ford Racing ProCal handset, an electronic tuning device that lets you adjust things like throttle response, ignition timing and so on.
The tool costs $595, which is pretty reasonable in the world of performance parts. Not only this, but Ford Racing offers calibration sets that can be used without voiding your warranty, something most actual physical performance parts won't offer. You might feel cooler bolting stuff on, but Focus ST owners can apparently get an extra 90 lb-ft of torque out of their EcoBoosts with this tool. There aren't official numbers yet for the EcoBoost Mustang, but as it is a bigger engine, we're confident that it will be quite a noticeable gain.