I hate to say it, but I think it might not help performance, and might even make it worse 
It would be worth taking a good example BP and making an instanced version by hand.
Then try putting the BP version all over the map, vs, the HISM BP and compare performance. Because, for example, the instancing is only within the BP, not over the whole map.
Also, if you really want to take this route, it might be better to use ISM rather than HISM. HISM has several limitations, such as culling and LODding in chunks, which ISM does not. These don’t matter with things like foliage, the main use, but might give surprising results with buildings.