The solution would address exactly that issue.
Every block needs to have a property that indicates which sides of the voxel it claims. Including concrete blocks in your example, which will claim all 6 sides. So they cannot share because they can only be placed in a block where all the sides are free.
But I agree - the more finely grained the world is, the more objects that can be in it, the more work the computer needs to do. So this will definitely have a performance impact.
But I dream of such things being possible if TFP could optimise it enough.