Yes, a good-quality tuner will improve the horsepower of your engine if you're driving a Mustang. A realistic range would be around 12-35 horsepower/TQ. This might not translate to mind-blowing performance, but it's apparent. Although the additional HP might not sound like a lot, you have to consider the fact that this is just unleashing the true potential of your Mustang. It's not built for dramatic changes.