It was fine up until last night when I discovered the planet wasn’t ticking 
Ouchy wouch, first big performance hit of the project. On the up side, finally now, the real work can begin.
The good news is that what has been running breezily up until now, is UE4 rendering the amount of procedural meshes I needed it to (if that wasn’t working it would have been a major blockage).
So the next bunch of work will be all about slow grindy methodical testing, debugging and optimizing of the mesh calculations and generation. From there, I’ll have a much more accurate picture of what UE4 will and won’t be able to do as far as planetary terrain goes.