For some odd reason, when I first tried to download the .blend, it opened up on a webpage as binary gooble-de-gouk.
Strange but my point is this; it says that you are using Blender 2.64 and not that long ago there was a change made in that normal maps are now rendered differently so that seams do not show as much. You might want to upgrade unless you have a good reason for not doing so.
It has something to do with normal maps being rendered with values that were slightly lower than neutral colors which was causing seams to show when mirrored.
In fact, I think it was @metalliandy who pointed this problem out to Brecht on the bug tracker.
BTW. If you feel like sharing your technique for creating normal maps without baking I’d also like to know how you are doing this.
The only thing I can think of is that you took a screen capture of the uv’s, then applied this to a subdivided plane and sculpted it, then used the ‘normals’ matcaps to display it, and then used the OpenGL render button to capture the normal map.
Well, anyways, if you feel like sharing, feel free.