I'll just provide a short answer in regard to "where we're at". We've run into some show-stopping performance problems which have their basis in ACS 4 Classic's dependence on certain Oracle optimizations that aren't mirrored by Postgres.
Until we fix these problems - and they are fixable - scalability testing on the Postgres front, at least, is meaningless.
Your comment about needing a third tool (SQL Relay) in order to effectively implement connection pooling in the Apache/PHP environment is interesting, I didn't realize it was such a PITA in that environment. The things we take for granted as AOLserver users :)
