Looking at getting the AF Dynamic brand cold air intake for my 2013 Impala. I've read about some of the benefits but I'm mostly interested in gas mileage improvements. On the way home from a festival yesterday I reset the gas odometer and heading down I40, I95, and then about 30 minutes through some back roads I averaged about 34 mpg. That was an 1 1/2hr trip. What have you guys gotten with your mods?

Scotty makes some good points about Cold-Air Intakes on modern cars

^+1. Cars come factory with cold air intakes....

The best point he makes, all you really get is more air, and since the computer is not tuned to make use of that air, it's really no benefit.
Let me add, some people in here have invested in HP Tuners. I think it's about $500-600 for the software, the cord, and a license to use for your car. But you really have to know what you're doing to go messing with the ECM, or TCM.
There is also a guy on here, Overkill, who sells a stock tune for 87 and 93 octane. Once purchased, you can use the HP Tuner software to send it to your car.
Many have praised the improvements, although at that price, it may not be worth the the investment of the tuning software.

But, if you know your stuff, you could use it to tune your computer to make use of the extra air.
Still, that's alot to know, and alot to pay just to be able to actually gain any benefit from the increased airflow from a cold air intake kit.

The GM engineers look at something called volumetric efficiency which decreases with RPM, the higher the RPM of the engine the lower the volumetric efficiency due to the fact that the piston is moving down so fast while the intake valve is open it creates a vacuum in the cylinder and when the intake valve closes that vacuum has prevented the cylinder from getting a full air/fuel charge compared to the cylinders calculated volume.
Several things in the intake tract have an effect of volumetric efficiency including the design of the intake, the air filter it's self, design of the head and size/configuration of the intake valves.
Engineers look at all this with the head and intake valves commonly being the largest restriction, they make sure that the air flow thru the stock intake and air filter at least matches the maximum flow that the heads and valves will allow at maximum RPM, therefor increasing air flow thru the intake and air filter will have no effect at all on engine performance as it is not the intake or air filter that is restricting the flow.
The only reason people think these cold air intakes increase performance is because they are louder and cause people to think their car is faster but it's only faster because they are using more throttle because it sounds good, in real life on the majority of vehicles they actually decrease performance due to feeding hot air rather than cool air due to the intake being in the hot engine compartment rather than sucking cooler air before it has gone thru the rad and AC condenser.