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If you're not wanting to spend that much, 5 Star tuning can provide some performance improvement with a canned tune, although I have no idea how much SOTP improvement you'll see.
Anybody can build anything, with the right budget. What's your goals, and how much are you willing to spend?
I'm not so much worried about the HP or torque as my main focus is on improved fuel mileage. 2-3 mpg more would be awesome. I'm getting ready to purchase a 6.7 diesel however I would still love to increase the mileage on the 6.2 seeing how I plan on continuing to run this truck as well.
The only mod you're going to do that will improve fuel economy is the driver mod. Avoid powering up to stops, rapid acceleration, and high RPMs. Remember that your brakes are bleeding off energy bought and paid for with your fuel dollars. Driving habits can make a 25% difference in fuel economy.
Tires with less rolling resistance would also help, but the bottom line is that you're driving a vehicle that weighs over 3 tons and has the aerodynamic properties of a house.
Single best MPG mod is slowing down on the highway. Run 60 instead of 70, you'll see huge gains. Same for gentle acceleration. Drive like there is an egg taped to the surface of your gas pedal.