You dont to need modify the computer to adjust timing and fuel pressure. Taking it to a shop just for those reasons would be a waste of money, because it can be done in your driveway. You can adjust your fuel pressure, only if you have an adjustable fuel pressure regulator. The stock regulators arent adjustable.
A custom tune (i.e. chip or stand alone) changes the tables stored in the computer. These tables tell the computer what timing and fuel should be under a given set of conditions. By having these tables adjusted to fit your cars needs you will make significantly more power and drive ability will improve. If you are going somewhere to just have your fuel and timing set, that is not worth it, but if you are going somewhere that has the ability to custom tune the stock electronics, that is a different story.