Dissasemble the intake from filter box through upper plenum. Clean it all out, use proper cleaner on the MAF sensor and in the throttle body. Or have a shop do all of that.
Get a K&N drop in replacement filter (high flow). Fits into the stock FX and never gets replaced again, just cleaned once or twice a year.
Definitely stick with premium - 91 or 93 Octane. On your next tank, do some fuel injector cleaner in the gas tank, and I've never tried the seafoam cleaning system but hear good things about it.
All of these things should show improvement during the first tank of gas, if you are driving normally. If it doesn't, then I would look into a tune. That seems like really low mpgs for an FX, unless you are really agressive.
After hitting 100,000, I did the intake cleaning that I describe above and saw an immediate 2.5 mpg increase. I'm on my fourth tank of gas since the cleaning and my computer says 19.0 mpg.
