Properly matched bolt-ons and a good tune can easily gain 50-100 hp. People would not be spending their hard earned money if it did not. Things like gears are included here.
It does cost more money, but a Procharger is also a true bolt on. No pulling the intake or anything like that. Easily done in a day. Magnacharger is not much more difficult either. But with FI mods you really want an updated exhaust to maximize the increase.