Ok, because i was interested in this topic too, and cuz i never used instanced (but wanted too, especially with Nanite), i tried it for myself, to see, how bad performance tanks with Nanite and instances (not Hierarchical instances, because i think, i let Nanite deal with the LOD stuff and triangle optimisation).
So one actor with an an array with 256x256 =65536 instanced Nanite Cubes, no FPS loss other than during creation:
Just wondering, i tried it with 1024x1024, but for some reason, it didn´t created all instances, it stopped about halfway through. Has Nanite a limit about how many instances it can support? 