I think you might be getting to the limitations of AA and sampling in general. If a fence is one half of a pixel thick, you need at least 2x SSAA. If a fence is 1/4th of a pixel thick, you need at least 4x SSAA. If a fence is 1/32nd of a pixel thick, you need at least 32x SSAA, because 16x SSAA will look ugly. And I could go on...