the modeler I work with has poor fbx support, and poor smoothing options, for smoothing I have to create extra material elements for each different smoothing angle, even if the whole object would otherwise only need one material
my only other option is to unweld vertices to modify smoothing, and I’d rather not do that
I’m just wondering is there any performance cost to all these extra material elements even if once in unreal I put the same material on all of them?
Unwelded vertices should incur certain performance cost themselves. As for different meshes with same material, As long as they are static, there should not be large difference.
what I’d like to know is if a single mesh with multiple material element IDs all filled with the same material would have any performance cost over the same mesh with a single material element ID and material
Each material element will invoke a drawcall, even if the materials contain the same data/instructions.
Do you work with Modo?
I work with it and there the smoothing group handling is the same.
What I tend to do is splitting the normals at hard edges.
Another way would be :
Apply the various materials in the modelling application
Import to UE with all the material slots
Use the Merge actor tool. (Yes, it also works with a single actor, merging it with itself)
The resulting proxy mesh/material should then only have one material slot…
urgh that’s what I feared, though tbh even if there was no cost I would have hated having to place the same material over the mesh lots of times, so maybe it’s good I have to find a better way
I work with lightwave, so that probably explains the similarities, though unfortunately there’s nothing I can do about it
btw though I thought modo now has smoothing groups similar to max:
this one sounds interesting, I’ll try it
Just using Modo recently. 
But good to know
ok this works! but on closer inspection I don’t think it’s feasable for this project
after importing I have to go through the meshes and set matching materials into the duplicate elements for them to be merged, then the merge actor window displays every actor in the project and I have to go through the list and uncheck everthing each time before I can merge a single object, then repeat again for dozens of objects
and then if I want to edit the original geometry later it won’t auto update in ue4 because the mesh is saved to a new asset
alas looks like I’ll have to find another way
I had experimented briefly with it. I thought the materials were combined when I chose “flatten materials” or something like that.
Ill have to check again
That is where a studio would hire some student temp workers 
Or, where the single indie dev spends a lot of time.
Remember, this an asset/game polishing step. Clever setup of the HLOD clusters will also take you some time as there isnt a single correct answer for each case either.
Indeed. The merging step would be the last one in the pipeline.
Leave the mesh multi-materialed until its really really finalized.