* Existing Intake System: The GTO's factory intake system isn't terribly restrictive. The potential for improvement is therefore less than on a car with a more choked system.
* Quality of the CAI: A poorly designed or cheaply made cold air intake might offer no gains, or even a slight loss of power. A well-engineered system will be more likely to produce a noticeable, albeit still modest, improvement.
* Tuning: To fully realize the potential of a cold air intake, you often need a corresponding adjustment to the car's engine management system (tuning). Without a tune, the gains will likely be smaller, or you might even experience slight performance dips due to the air/fuel ratio being off.
* Other Modifications: The gains from a cold air intake are more significant if you have other performance modifications, such as an exhaust upgrade, headers, or a tune.
In short, while a cold air intake *might* add a few horsepower to your GTO, don't expect a huge jump. The 5-15 horsepower range is a reasonable expectation *if* the intake is high quality and you get a proper tune. It's more likely to improve throttle response and perhaps offer a small increase in torque rather than a dramatic horsepower increase.
