The Anti-popup and Altitude cull only affects 3d objects, not ground textures.
Regardless of how high you set the Anti-popup slider, the objectss will never be loaded before the "global" area of the airport, which is 10 nm, which is more than enough to be realistic for an approach, without causing performance and/or memory problems in nearby airports.
The grey area you see when the objects and the ground disappears, is the AFCAD background, which has an higher range and is set independently by FSX.