Its not a matter of if they add power or not, its a matter if they add power over the stock Mustangs air filter box/tube. Ford did a really good job in the design of their stock air intake system. Flows as good (and sometimes better) than most aftermarket CAI systems. Some of the aftermarket systems have an open element; it's not inclosed in a sealed box. The open element ones simply grab warm underhood air, which actually is going a step backwards over your stock unit. Some have a shield that supposidly seals against the hood, but they still draw warm, engine heated air rather than cooler air that a stock setup does.
Most folks buy an aftermarket CAI to "dress up" the engine compartment, or to obtain the "roar" that an aftermarket CAI can provide. Other than that, they really do not give any seat of the pants performance gains on their own. Should you want to hear the roar of the drive by wire pedal when its stomped (formerly known as a gas pedal), you can mod the stock air box; remove the sound deadning box thats on its side. If you seach here, you'll find out how to do this.
Some of the more expensive CAI are sold with a tuner; these provide better numbers, but it's because the tuner/tune thats sold with them give the performance boost. These CAI's have a slightly larger tube diameter over stock, so they have the potential to flow more air, which means the PCM/ECU requires re-mapping of the fuel/air ratio, hense the requirement for the tuner/tune.
It is better to keep your mouth closed and let people think you are a fool than to open it and remove all doubt.