I have exactly the same problem…rock mesh sculpted in zBrush, lowpoly made with TopoGun, Normalmap baked from the highpoly in Maya. As soon as I import the rock to UE4 I have those ugly seams.
How did you manage to invert the green channel on the normal map? In the UE4 TextureEditor I don’t see this option…please help me out in case if I’m just blind like a mole.