The further apart the ships are, the less difference height makes. With normal ammo, the artemis can hit something a little over 190 meters above it at max range (a little over 390 meters above it when using lesmok with the new max range). Since the angle is fixed, the closer you get to the ship, the less vertical distance it can cover.
In a sniping match, range will always trump the turning angle. Even something like a field gun can cover a huge area when the ships are far apart.
When you're up nice and close (400 meters), the bottom of your ship needs to be at least 70 meters above their gun to avoid being hit. Get to 100 meters and you just have to be 18 meters above their gun. Of course you have to keep in mind that things are always more complicated. Ships are volumes, not just points in space. The mobula for instance has it's guns at the top of the ship, not the center or near the bottom, giving them a little vertical boost. Not to mention the height of the spire.
All maps also have varying distances between the floor and ceiling of the map. Terrain factors like hills, valleys, and objects make a difference. Doing some testing I found that from the deck of my ship to about the bottom of the map on dunes was between 450 and 600 meters. Probably a little more and a lot less in other areas on that map.