Does the lowpoly shading look good with the normal map applied? Hard edges are certainly useful to control the shading of the lowpoly, especially at steep angles - just remember to split the UVs along hard edges to avoid artifacts! Maybe check out Making sense of hard edges, uvs, normal maps and vertex counts?
I think object space normalmaps don't use the normals of the lowpoly at all just override them but tanget space works together with the lowpolys normals.