Wow! Very impressive. I wonder what's the size of your file. Even with minimized shaders it must be huge.

But I am also wondering about replicators: I see you are using some (judging by bounding boxes in fly through), but are the smaller models there in instances or are they also replicated all over? I wonder whether it would improve the time/storage/memory to use lots of distribution maps and replicators with few actual instances, or whether with really low poly models it's actually resource-cheaper to use them instead of the maps?