Eliminated acs_privilege_hierarchy_index in favor of using acs_privilege_descendant_map. In addition, rewrote trigger for creating acs_privilege_descendant_map as permissions creation was getting too slow on .LRN. .LRN has somewhere in the neighborhood of 90 entries in the privilege hierarchy, which is much higher than anticipated.
Added code to make use of David Walker's wonderfully abusive hack on the pg_proc table that showed me how to write recursive SQL functions. We now have good use of the index on tree_sortkey when climbing the tree, woo-hoo!