For most craft you will not notice much differance.
Gun harmonization adjust where the lasers converge. For most craft, this is done automatically. If a Targeted enemy is .5 km away, the lasers will converge at taht point.
For some craft (like the z-95) you need to constantly adjust them (i tend to keep it off).
A good test to do, is find a container, move up close to it, and come to a stop about .05 km away. Target the object, then move so it is directly behind you. FIre the lasers with differant settings so you can learn what it does.
For small targets i normally ahve it on. but for bigger targets, or very close in fighting i turn it off, tend to be more accurate that way