Well, I spoke too soon about the fast normals. They almost work. The normals do not line up exactly at the corners of the cube so there are visible seams. I spent some time trying to come up with some extra rotation to compensate for this distortion but was not successful and have decided to revert to calculating the normals the old way, which is by calculating triangle normals and then smoothing.
I believe to fix the cube to sphere mapping you need to rotate the normals about their vertical axis as they approach the corners, but I have not come up with a good method yet, if anyone has any ideas I'd love to hear them.
(click the above image to see the normal map corner distortion)