I have played with those settings before and they never really made any difference. But then I don’t really have any reference to what Start Height/Height refers to. Is it height of the ? Height of the terrain? If so is it the height of the terrain below the waterline? Is it the heightmap? And if it’s refering to the heightmap, shouldn’t the values be between 0.0 and 1.0 in that case?
I would never use a JPEG though 
Oh man that would be awful.
What I did try once was to use an 8-bit version of my heightmap at a lower resolution. But that just crashed UE4 everytime I tried to run the game. That made me think that the settings are only applied once you run the game, but when I checked (with the proper heightmap), there was still no difference with an updated heightmap. It was as if the old one was still being used.
So I’m still confused 