I worked at K&N at one time... you DO get more HP. even if its a high millage beat to hell engine, it will gain HP.
but you will loose economy 90% of the time
particularly if it goes on a OBD2 controlled motor. with all the extra sensors involved, more air = more fuel in the engine.
making the engine turn easier is the key to HP and economy, yes. but, with newer motors it is very difficult to fool the paired engineering and computer programming to get you better gas millage.
i mean, you dont even get to drive new trucks now. the gas peddel is a controll stick, which tells the engine throddle to open... you dont actually get to open the butterfly anymore...
Electric fan (over mechanical), long tube headders, high flow muffler, and an isolated cold air intake will get you a lot more HP, but with an OBD2 truck you may not see much improvement in fuel economy.
simply because the engine's driving habbits will not change. even programming chips/devices can only change the computer & engine's operating habbits for a little bit before the sensors reprogram