Do you track how far you drive on how many gallons?
-16 Fuel Trim is what the readout says. how do you know for sure that your're not beyond it? I guess it all depends on your "bible" or belief system...but if your mileage is up over stock/government ratings, you're doing well. if you're consistently doing significantly better than them, you're doing awesome.
Also remember that computers are stupid things, only doing or telling us what they've been programmed to, and the result may not be as expected.
I'm working on getting my Pilot's license, and we use trim in airplanes too. There is a range to how much trim we can apply, but the result depends on certain factors: X degrees of forward trim may work at Y RPM to maintain level flight on some days, and on others, you may need Z rearward trim at that same RPM, depending on winds, atmospheric pressure (MAP sensor), air temperature (IAT/MAF), humidity (MAF)... (sound familiar?)
Don't get stuck on the trim number, get stuck on the "distance to empty" or "fuel range" a lot of cars display. That's what you want to increase. Trims help diagnose problems, but the be-all, end-all is distance travelled on a tank/range/runtime. just because a scantool is saying your trims can go +/-20 and it's only showing -16 doesn't mean you're not already trimmed out to the max and fooling the computer hardcore.