the main purpose behind aftermarket intakes for our truck isn't for "cold air" as the OEM intake pulls air from the fender well. They are designed to provide the least amount of restriction and turbulence.
When you start upping the horsepower and amount of air moving through the motor, you will need less restriction.
The claims of the K&N 'losing' power is because it's an open element intake, meaning the intake is prone to pulling hot air from the engine bay rather then relying on only cold air from the fender like the AFE or TRD style intake.
When you are cruising though, the IAT (intake air temp) of the K&N is maybe 1-3°F warmer then ambient. You won't see a performance difference between one intake and another with the IAT that close.
As for gas mileage, as long as the intake you are running has good MAF accuracy, and you keep your foot out of it, you won't notice a difference.
The K&N and TRD/AFE all have great MAF accuracy (as documented through data logging).
In order make horsepower, you have use fuel. More power = more fuel. But when you have your foot in it, i REALLY doubt the thought of "I wonder if i'm getting good gas mileage right now" goes through your head.
A tuner like the URD
Maf Cal is great for most applications for making sure the AFR is where it should be for optimal performance. For complete control, and max performance something more advanced like the APR X-1 will be needed.