Doesn't help economy at all with a fuel injected engine.
http://www.fueleconomy.gov/feg/maintain.shtml
http://www.fueleconomy.gov/feg/pdfs/Air_Filter_Effects_02_26_2009.pdf
Carbed engine it would help since a clogged air filter would affect mixture.
As far as how much dirt passed with a K&N this test shows the K&N passes a heck of a lot of dirt vs others.
http://home.roadrunner.com/~jbplock/ISO5011/SPICER.htm
Bottom line...use the stock AC filter on your truck if you want the best filter. Use the K&N if you want the most dirt in your engine. I used to use K&Ns. I don't anymore. I personally found way to much dust after the filter in my intake system and I had not even read this test yet.
An engine is basically an air pump. If you want more power you need to figure out how to make it pump more air. You can slap on any number of things and it won't help a bit if there is one thing that is preventing more air to go through. That is why Vortec heads were so important for creating more power in these engines...because they flowed air better. Changing just the exhaust or just the headers or just the intake won't do anything if the heads aren't flowing at their maximum operating peak for example. People spend a lot of money on aftermarket ad ons for their vehicles and the only thing it does is make their wallet lighter and give then an emotional feeling that since they spent money that somehow it has to run better.