1. Postgres doesn't provide any way of downgrading, although it has extensive support in pg_dump for upgrading.
2. The slony replication system has been used to downgrade. And it supports postgresql back to version 7.3.x. See http://gborg.postgresql.org/project/slony1/projdisplay.php
3. Have you asked in the postgres mailing lists about your performance issues? There is a list just on performance, as well as the hackers list where all the postgres developers hang out. See http://www.postgresql.org/community/lists/
