Couple of suggestions: Increase lightmap density on your object. I see it's at 256 now, so I'd put that at the second option. The first thing to do is make sure you are snapped to pixels in your unwrap. In your example, use a 256x256 checker and make sure all edges are on pixel edges in the unwrap. In UDK, re-bake at production level to check it out.
Let me know if that works for you!
Try selecting the coplanar surfaces that share the same texture, right click and then Alignment -> Planar wall. This worked for me with floors, although I tried also with walls and it didn't U_U. Hopefully it works for you.
The first thing to do is make sure you are snapped to pixels in your unwrap.
This was the step that I had missed and it improved the lightmaps A LOT. There is still a slight seam in there but I'm thinking this might be to do with lightmap resolution now. Is it possible to have a completely seam free wall? I'll investigate further and post back any progress.
Is there somewhere that you can alter how lightmap LODs are used? It get's noticeably worse from 10ft or so away!
In BaseEngine.ini look for:
and set it to False. This will force the highest level of lightmap mipmaps to be used regardless of distance. Helps a lot in my experience. In theory it hurts performace, but quite honestly i havent seen a decrease in fps at all.