First up, welcome to the site.
Next up, please try a search- all of those questions have been answered at least a few times each- not being rude, just pointing out a fact.
To re-visit them:
You MAY see a slight loss in fuel economy from a mild lift. Most of that is found when you level out the front, as the factory rake (nose-down) helps reduce airflow under the vehicle, helping aerodynamics some.
If you're not looking to add exhaust noise, adding an aftermarket exhaust probably isn't the route to go- they all add at least a fair bit of noise. You can try going to a muffler shop and having them fabricate something. The end result with any of them is about jack- they reduce weight a bit, and the right muffler will help ground clearance at the rear (the OEM mufflers tend to be rock magnets for some people), but power wise, nada. Zip. Zero. It's all a perceived gain, none real that I've seen. Fuel economy is probably the same, although it may be worse for some who enjoy a more sporting sound, and tend to romp on it a bit more
As for Mygig, that is included in the OEM nav system, so I'm not sure what you mean there- Mark W.