Can I ask you a question... why 3" why not 2.5"? What are your power goals?

How much you gain will be solely on what type of setup you run... every car will perform differently depending on what add-ons/mods you have. You'll feel a difference but then it a few days it'll feel like its just normal. Couldn't tell you what kind of gain you would get... every car is different and modded differently.