You can buy off the shelf tuners that are 'canned' tune. That means they just have a generic, preset tune that may or may not give you the performance gains you want.
To really get the best bang for your buck, your going to want to get a completely custom tune that factors in your specific engine/trans, gears, tires, any bolts ons you may have, the area you live in (i.e. elevation, gas quality.. yes, gas quality changes), your driving habits etc.. etc..
Buying an off the shelf tune is just an 'assumption' made by a dude who borrowed a vehicle and put it on his dyno. Sure that tune will work good for the test vehicle, but will it for all the other people who buy it?
I may be incorrect, but the SCT X3 was designed to allow people to really open up the computers in their vehicles and adjust and fine to everything. The SCT X3 isn't just something you plug up, flash the tune to your vehicle and go (that's not to say that people aren't doing it this way). If you don't have the knowledge to use the SCT X3 to tune your truck's computer, then you need to get someone to do it for you.
Think of the SCT X3 as a tool used to tune the computer in the vehicle, I think your thinking of it as like chip that you can just plug in.
