Hi Hannes,
Your projection settings are correct.
The issue is that the supersteep displacement causes amazing stretch in the surfaces, as you explained.
The compute terrain needs to compute surfaces and UV/texture coordinates for this surface, but with a very large patch size it probably won't calculate the sudden steepness of the terrain correctly, resulting in these massively stretched textures.
What you need, and I tested this with your file, is to give the displacement a slightly soft edge to allow more polygons to be created over the patch of surfaces.
Consequently you need to make the patches for calculating these surfaces smaller and thus you would also need to decrease the compute terrain's gradient patch size.
So I have set the sss's softness to 0.5 metre and decreased gradient patch size of the compute terrain node to 0.1 metres.
Now my texture maps much much better.
This isn't really desirable though if you do much displacement before this node or especially when you have computed the terrain before this compute terrain @ 0.1 metre gradien patch size.
Displacements can be more tricky to control and/or render times will start to go through the roof.
So at the moment this seems the only way to do it.
You may find one of my recent feature requests in the alpha forums interesting as it deals with this problem.
However, the method I describe doesn't overcome this issue directly, but can be at least a nice improvement.
Cheers,
Martin
