Originally Posted by Josh@STILLEN

If you are going to a Pro Tuner and having your car tuned you're still starting from the base tune in your Z, and adjusting the values from there to dial in A/F ratio, and of course could play with some timing to extract as much HP as possible.

It's not throwing your existing tune out the window, it's adjusting what's there. There are tables and portions of the ECU which remain uncracked from UpRev, and aren't adjusted. There's a lot going on in the ECU.