Then there's nothing else you can do.
The way the simulator works, if a mesh is of an higher resolution, it will always show up on top of every other mesh, even those on an higher layer in the Scenery Library, under the assumption you would to always want to use the highest resolution data.
However, KIAH doesn't really use a photoreal scenery, in strict sense of the term, since a photoreal scenery cannot have enough resolution for an airport, so it uses normal polygons, so it shouldn't be affected by the normal mesh loading rules.
Instead of changing the texture resolution, try to change the Mesh resolution. Try with 10 or 5 meters / pixel, and not any higher.