I am using much the Hierarchical Instanced Static Meshes. Most of the house pieces and repeating details are instances. Recently began to use Instance Tool by Nate:
I would estimate I have few 100s instance component actors with total 1500-4000 instances (+ foliage). The performance is superb (55-70fps), and having no issues when playtesting. Unreal Engine is using 2647mt.
Been testing with only 3 levels active while having this crash. Having 17 levels active have been working while playtesting.
EDIT: Restarted editor and now it says Unreal Engine is using 10986mt memory. Was already 9800mt before loading the 2 levels and 1000mt after those were loaded. :S