I've always had this problem with Corona Renderer / 3ds Max, and have basically avoided it by using high poly geo whenever I can. When working with game assets, however, this is unavoidable and produces very harsh shading artifacts as described above.
There will be times when it is not possible, or practical, to get normal maps for an asset baked in [whatever tangent space 3ds Max prefers].
I'm wondering if CoronaRoundEdges or the new PBR shader will have any say in this?