Let's try and erase the "how much power will I get from a tune?" question much the same as black people tried to bury the N word a few years back. That would be great.
Smartash comment aside, don't think of the tune as a power-adding mod. Sure, you can definitely gain power, but try to think of it like this -
A company has several employees, all doing a certain task. Without a manager to oversee and manipulate daily activities, the output is inefficient. Stuff will get done, but maybe a few of the employees are underutilized and can handle more work. Maybe some are overworked and are freaking the F out trying to make up for the slackers.
With a manager, each employee can be monitored so they work as efficiently as possible, thus maximizing output of the company in a safe, reliable manner.
Now, Im sure you know that the manager is the tune and employees are the mods to the car. I see a tune as the glue that bonds the whole project together and is imperative to have a safe, reliable car that makes as much power as the combo is capable of. Hope this helps.
And btw - I had my '99 retuned the other day. I went from ~420 whp to a hair over 460 whp just by adjusting the timing and fuel curves. The tuner and myself alike were impressed, to say the least. Money well spent.