I tried to increase the amount of faces in a mesh to see how high poly effects performance. But even though the mesh becomes incredibly dense, in shader complexity mode(ALT+8) it still shows it renders fine. So I’m wondering what cost of performance will high poly have? or is it because this mode can’t represent the cost by polygons?
Mesh complexity has no bearing on the shader complexity view mode. Modern GPUs can push tonnes of triangles, so you shouldn’t really worry too much, especially if you use LODs. Look at Megascans assets for a good idea of acceptable densities for different types of mesh.