Josiah,
This seems ambitious. The only advice I can offer is to set your min_level (on each layer) to some number higher than the default (which is zero). osgEarth is trying to build LOD 0 by compositing 800 layers -- and you probably don't need to see elevation data from orbit anyway. Try min_level="7" or something like that and see what happens.
Glenn Waldron / Pelican Mapping