the stock setup is the best for efficiency. i use the k&n drop in type filter and on trucks its not much of a increase. the only power those intakes make is above the usable range for normal driving

Final drive ratio makes a difference, too. I have an 01 Silverado 5.3L, but it came with the HD tow and Z71 packs, which gave it a 3.73 geared diff ratio. Other trucks out there only got a 3.42 ratio. Around town, it probably doesn't make for a huge difference. But the ratio will be noticeable with highway driving.