I have always used a drop in K&N just to be able to wash/reuse it. I would be shocked to see an actual MPG increase. Normally more air=more fuel, but I could be wrong. An all out cold air intake will add better air flow for sure which means more hp, but better mpg? Seems like you can have one or the other, but not both.
Did the Tundra have anything else done when the cold air intake was installed? Maybe a MASS cleaned or something that may have bumped MPG up? If you find out a simple air filter will add 2 mpg, I'll be the first to buy one. That would be awesome, just not realistic I'm afraid.