The VXGI lightmap baking option is purely hypothetical at point, as far as I know. is a known request, but the implementation in UE4 is probably nontrivial, although VXGI has the facilities required to implement it. Doing that in runtime is even more complicated because lightmaps are static in UE.
If your scene is mostly static, VXGI integration can be modified to skip most of the scene voxelization on every frame. For simplicity, the current UE integration configures VXGI to invalidate and update everyrhing on every frame, but that’s not absolutely necessary. For a static scene with no multi-bounce lighting, both opacity and emittance textures can be preserved between frames, with only incremental updates caused by camera movement. Implementing mode requires building a list of objects and lights that have been modified since the previous frame, and applying finer culling to the voxelization passes.
Regarding performance improvement, if you observe that “VXGI WS” time in “stat unit” output is a significant part of the frame, then yes, using incremental voxelization should give a noticeable improvement.