Maintenance of the tree_sortkey structure is expensive, that's one reason not to adopt it in all of our Oracle code. You are right, though, that the sorting issue is a PITA in Oracle.
OF did take the tree_sortkey approach in the forums package, I helped to the extent of digging out the right datatype to use (RAW). I'd like to see us do some real-world performance testing of insert/update on large tables using this technique before getting too enthusiastic about using it. It may be that the penalty in Oracle won't be so bad (PG is just downright slower at massive inserts and updates even without the additional tree_sortkey baggage).
