While each vehicle will be different, we are always aiming for more power, more gas mileage, and the best driveability possible. Gains will depend on what kind of engine you have and what modifications are done to the vehicle. For instance, a car with headers may pick up more from a tune than a stock vehicle does. Most of our tuning descriptions will tell you what kind of gains we expect.
From the factory, GM often leaves a lot to be desired in the calibration. They are set up on the weak side because they do not know who will be using what car for what purpose. Since you know the intended use of your vehicle and the modifications you are planning to add, we can nail down a file that will work for you based off of our experience. In the tune we can change things such as timing, fueling, power enrichment, rev limiters, speed limiters, speedometers, VATS (security), shift firmness and time, torque management, and much more.
Getting the PCM tuned with the required settings, will get rid of the rev limiter 
Maybe you needed to specify it in your order, I dunno. But that's the only way to turn it off, is to program the PCM not to do it...
