Welcome to Ramforum. Your basic bolt-ons will show slight gains in mileage. I would start off with a cold air intake, ported throttle body, and a free flowing muffler. You should see about a 1.5mpg increase with these mods.
Intakes - Your 2 options for the 2013 are the Vararam and the S&B. The Vararam will show the best all out performance gains, but the S&B comes in at a close 2nd with excellent build quality.
Ported Throttle Body - Our 87mm throttle body has been tried and tested by hundreds of members here. It is going to show an instant bump in throttle response as well as an easy 10-12hp gain. It also installs in just a few minutes making a tough mod to beat.
Muffler - I would replace the stock muffler with a better flowing Magnaflow. Magnaflow mufflers have a straight through design that offer the best combination of performance and sound. You have multiple options depending on how loud you want it to be. The shorter the muffler the louder it will be.
Once these mods are done, I would start looking into a tuner to help tie everything together. As of now, Bullydog is the only company offering tuning for the 13's, but they are still working out a few kinks in the tune. I would hold off on this until they get a few of the bugs worked out. Diablo is also saying they are within a few weeks of releasing their 2013 tuning, so you will have a few options to choose from.