Some general information.
The FCC is somewhat sporadic in their enforcement. Last November they went after 10 unlicensed stations that exceeded the signal strength limit of 250 microvolts per meter at 3 meters. One was only about 100 microvolts over the legal limit.
15 watts of RF power at 50 Ohms is almost 30 volts of signal, a fair amount higher than the legal limit (measured in millionths of a volt).
The location of the transmitter relative to others that might want to receive a weaker station can play a part.
Not all of these imported transmitters (they are likely not FCC type accepted) have the cleanest RF output. If one is within a mile or two of an airport, it would not be a bad idea to use caution.
Radio Locator has a function to help one find possible empty frequencies, likely a good idea.
Some additional information. A top of my head calculation indicates that the above 15 watt transmitter at full power and the antenna on a ground plane in the open (on the roof a car for example)...
Will produce a field strength in the ball park of about 9
million microvolts (9 volts) per meter a 3 meters.
It might not be a bad idea to pay attention to the location of the transmitter antenna with respect to any of your receiving antennas.
Even with 15 feet of separation, the field strength could still be several
million microvolts per meter. This much signal might not make some receivers happy.
It is not a bad idea to start at the lowest power level and increase the power level only if needed.