There’s no good automatic unwrap tool, most likely on a mesh that high poly it will be really really terrible results anyways. What type of thing are you doing that’s 500K polygons anyway?
I don’t know that DX12 can have any increase in the number of polygons, the main improvement is how it handles draw calls which is currently the biggest issue in games performance right now.
Also, as far as poly count goes in UE4, by default there’s stuff like post processing that’s on by default, which will significantly impact performance.