Blazo, bigger tires is going to cause your mileage to drop regardless of how big the rim is. As for whether 20" rims will be better than say 17" rims, that is really dependent on a lot of factors. You need to look at overall tire/rim weight. If you get steel rims, then your mileage is going to drop more than if you get say 1 piece aluminum rims. You will loose less if you go with say a 3 piece aluminum rim. Granted, a 3 piece rim is much more expensive than a 1 piece rim. pay me up front, pay me a little bit over the years. That is the question.
Also keep in mind that the bigger tires is going to result in a slower take off unless you put your foot into the gas more. Putting your foot into the gas more is only going to cause your mileage to drop that much more. Situations like sitting in line at the HRBT is going to kill you to with the bigger tires (I know first hand, got 37's on my Expy).
If you have more questions, let me know. For shops in the area, Pungo is probably the best that you are going to find when it comes to 4x4 shops, but I would not personally let them touch my truck. But, you can make your own decision.