from the papers I can’t directly infer if they stream components in place with proxy meshes or if they keep them all visible and simply LOD them. but I can agree that expecting 10km-long vistas on pure heightfield-terrain would be naive 
once again I’ve just been wanting to mitigate the cost of Tessellation though. since I started these tests everything was in a really good state with tessellation off.
anyway I made some more tests with my existing 8k landscape. I changed the sections per component from 1x1 to 2x2, disabled all pre-pass stuff and played a bit with the shadow settings to try to match the cascades with the tessellation. I managed to get a solid 80-90fps regardless of where I move or look at (with the landscape occupying the entire screen). surprisingly the key factor was the sections per component, and the 2 other things had quite a little impact.
at this point I didn’t compare the tessellated vs non-tessellated landscape but I’m sure the difference won’t be as big as in my previous shots.
for me this is already a very good baseline considering I have a density of 4x4 quads per square meter -at some point I’ll test with 2x2 density and a 4k landscape instead, so same world size but half the density-, and that I could reduce the view distance using fog so I can stream components in/out, and that 4.20 will improve the shadow part even more
of course some extra control over things would be welcome, so all the things you mentioned would allow freedom for everyone to decide which corners to cut. and of course “better features” might still come (virtual texturing is on the roadmap. UE4 4.30 maybe?)
also I wasn’t aware that the custom node isn’t available for Tess output btw, so thanks for the info. it will prove problematic when I attempt to use dynamic branching 
btw when you say “significantly better layer splatting” do you mean that proxy meshes would use a pre-baked texture rather than actual painted layers? or do you mean something else?