Your link on the first page is broken, so I'm not sure if this is the same issue, but I get giant normal splices down the midlines of my ships quite a bit. I haven't fully explored this, but I notice that the hard edge disappears somewhat if the background color is similar to the hull color. Given this, I believe that if you have a massively different background color from the hull color, the change in the rgb values is enough for it to attempt and render a beveled edge there. So I suggest expanding the border on the UV faces that share an edge with the mirror axis to prevent the seam from appearing. Continue the texture over the edge, so that even though it won't be drawn, no apparent change appears to the normal map and it renders it flattened out. Like I said, not sure if this is the right thing or me missing the point entirely due to a missing image, but worth a shot.