Thanks for trying it out and posting your results Tom.
This has been improved quite a bit in 4.5, meshes no longer need to be closed. Non-closed meshes may not produce the full amount of occlusion though, because they can fail to create a negative region in the signed distance field which would have caused occlusion.
The original 4.3 implementation is probably 20% faster in 4.5 and another 30% faster than that in what will be come 4.6. It’s still much slower than I would like though, the goal is for mid-spec machines to be able to run it.
Note that if you increase LightSourceAngle on the directional light you can amplify the soft shadows look, although it costs more.
Almost all leaking type artifacts come from the distance field being generated at too-low of a resolution. You can increase this with DistanceFieldResolutionScale on the static mesh. The default settings are quite conservative to keep memory usage down, the idea being that artists will increase resolution where necessary but most of the time it should work ok.
You can enable ‘Use Inset Shadow’ on skeletal meshes and they will create a per-object shadow that is composited in with the ray traced shadows, to get the best of both.
The Ray Traced Distance Field shadows doc will be published soon, probably right after 4.5 release.