Higher polycount means more tris, which in turn means more verts and therefore more vertex normals. Adding more polys (especially via a smoothing subdivision method like turbosmooth/subD) can result in vert normals that better fit the ‘ground truth’ (the actual curvature you’re trying to represent) but not necessarily. For instance subdividing in Zbrush without smooth or using something like Tessellate (tension=0) in 3dsmax will give you more polys but the normals share the same direction as the edge loops/verts that they divide. Check out the polycount wiki jonimake shared. It means you can get much better lighting on your foliage with very low poly counts. Noor’s version of NormalThief (if you’re using 3dsmax) is great for this.