[sammyg2]

This has pretty much been summed up. A K&N will work better than an old clogged up filter but will not outperform a clean stock filter on most cars (probably all of them).
It will let more dirt in and wear out your engine faster but most of us don't keep a car long enough for that to be a factor.
Most modern intake systems are efficient to the point where the air filter is not the limiting factor anyway.
Plus if you let in more air, the modern FI system adds more fuel to keep the mixture right where it wants. More air = more available power, but not necessarily more fuel efficiency.

Same with cold air intake systems. the air is colder, so its more dense, which = more air. More air = more fuel. Increase in power, not economy.

UNLESS the engine was so bogged down before you had to put your foot in it more which changed the mapping to a richer section and the new filter doesn't require you to give it as much gas to go as fast (I really doubt that).