I dont own one but my mom does and I do most of the work on it.
But we are thinking about droping a K&N air filter in it. In my Mark VIII we got 2mpg out of one also they are washable so you never have to buy one again. Also there are some horse power gains out of it but very little.

The engine has a huge air silencer in it I think removing it will give you gains in mpg and hp. it isnt that hard to remove and its easy to put back on I just need to find something to cap it off. It did in my Mark VIII. The sound from removing it if like my Mark VIII shouldnt be that noticable.