Yes I would strongly assume that in specific cases using world composition will decrease your performance. Imho this does not mean it will do this in any usecase (and you’re just talking about world comp and landscapes, right?). Would you mind posting the links to those sites, cause I didn’t found them.
So maybe all those tutorials on world creator and world machine about importing a tiled landscape into ue4 for world composition might be useless but I would be careful to assume so.
Interesting, I didn’t know this and since you talked about a single tile in world composition and later on about drawcalls I assumed you meant one single static mesh. How does the lod transition work with the static mesh? Does the whole tile change lod at once, do you see rifts between the static mesh tiles due to lodding? Cause the landscape makes this lod transitions smooth and you can change all those lod settings in the landscape settings (and you can also have different lods for the visual and collision).
But this means you can’t use landscape layer blends and the landscape grass node, right?
Ofc you could mask this black white for each layer and grass type and take shots of it, but with all this it seems to me you’re trying to reinvent the landscape system with less features and would have to redo all these steps again if you want to change anything, but ofc without any performance benchmarks I can only guess the pros and cons 
Also you might wanna keep in mind that you can use vertex painting for your static meshes instead of the landscape layer system (then you could directly paint on your meshes without the need to take images from your landscape). Ofc you would then store the data in the vertexes, and I’m not sure what that would mean for performance.
And yeah assuming you did this swap with an 4kmx4km map the benchmarks would be really interesting to see, but of course any landscape size might be interesting too.