I havent played your game(have been meaning to 
and will).
By ‘‘player placed meshes’’ are we talking about items, buildings? Are they skeletal meshes or interpactors?
I remember once that i made a building composed of 100 interpactors(stupid of me) and was animated to lift up from the ground as a elevator.Performance was killed as udk coudnt handle so many interpactors animated at once.Than i exported the building to maya>combined everything>and ended up with a building of 5 big chunks and the performance was fixed.Not sure if this relates here but its good to share the experience just to remove one thing from the problem.
So that stunt was a engine limitation.
Why is the Skel mesh tris so much higher in the scene with removed meshes vs the populated scene,different camera angle looking at other things?