I think the normal map in this shot is much bigger than it will be in game. It also makes sense why the diffuse doesn't match, you can bake normals at any resolution, but try authoring Diffuse that high.
IMHO... That Halo 3 shot above looks way, way less nicer than what we got in the final. The final game looked bloody fantastic at times (Of course, damn right shocking most of the time) but character art wise, the chief in the final was far better than in that shot. Mostly because he didn't end up looking like soembody…