We did try VCT, but with VCT any wall below 3 voxel footprint will leak due to bilinear filtering if not placed exactly axis aligned and snapped to voxel grid. In the 4rd clipmap we are talking here about 3*(40k/64)=3744 UU thickness, which is quite impractical for walls, not even mentioning things like doors.
Noise is indeed an issue and we need to solve it. In the meantime you can add more direct lighting or try sky leaking to make life easier for Lumen.