WP / WC for a 1.5km2 world??? Sounds a bit over-engineered actually. Although not exactly sure what the final actual performance goal is obviously. Usually as a developer, you shoot too high and then have to dial things back a little. Anyway, maybe talking about UE’s mesh capabilities would help here. HINT: UE is a workhorse, just as long as you stay well clear of UE’s Landscape / Terrain system. 
So you can have levels with 10,000’s of actors over 100’s to 1000’s of Km without killing performance. Nod to LODS / HLODS / ISM / HISM and lighter collision of course. Typically WP / WC / Level-Streaming are all a nightmare, with just so many gotchas involved. So if you don’t need to make your life extra hard then don’t. Levels with 50k actors spread over 1000km2 are possible without any of the above. 