Any way to manually make destructible object parts, e.g. from mesh part? (I mean if there is a way to decide how should model be destroyed instead of random cuts)

Do you plan even less terrain resolution? It have quite huge affect on fps.

destructible objects are just actors + joints with breakable option, so you can build your own destructible classes which use on ready Mesh parts

you can set heightmap resolution, and also combine it with heightmap\mesh quality\quarter

I know I can set heightmap resolution but I'm asking if you plan adding smaller resolution (for now 32 is the smallest).

I don't think so, isn't 32 with quarter quality small enough? That gives you effectively resolution of 8.