Power Tune?

Custom tunes adjust many features in the computer of the car such as advance or retard the timeing so the octane of gas you use will match your tune. Check your local station to see which three octanes they provide then order your tuner. Do your mods first such as cold air and exhaust before you tune and some places you can get tune and cold air packages together. Since these cars are "fly by wire" it will take the lag in the gas pedal out.
Why Ford dont tune to the max, that can go many ways Im sure they dont want to give you a vehicle the burns the tires at every take off for one reason. I dont run my 93 tune just for this fact and I drive a 6r.
A Custom Dyno tune is when you take the car to a shop that does such tuning with a computer and custom tune you just order with a tuner or have emailed to you and you load it into your tuner. Hope this helps.

Yes It does void warranty and yes it does save your stock tune. If you return it to stock the they can still see that there was some alteration to the computer. It is said that after you return it to stock then disconnect the bat for 15minutes it will be clear. as most things are use at your own risk and take your chances.

The only real risk in getting a performance tune is if the tuner is a moron. Most tunes you get with the tuner you purchase are taken from a large data base and maybe slightly tweaked to your specific mods. If you want to be safe with more power then you can just pick up one of the Ford Racing power increase packages.