Q:

How big is the performance difference between Oracle and PostgreSQL?

I'm wondering about how to scale a database. Currently it uses PostgreSQL. Would switching to Oracle be worthwhile inspite of the coding pain and expense? Or is PostgreSQL + more boxes a better/cheaper approach?

A:

IME there's not a lot between any of the major DBMS in terms of the performance that you can achieve by performance tuning. It's not just my experience Open Source PostgreSQL Trails Oracle In Benchmark, But Not By Much
There is no substition for finding any performance problems and fixing them both in terms of client side and database optimisiation.

A:

I think that what you're suggesting is logically equivalent to Things You Should Never Do, Part I.  This type of conversion is effectively a rewrite, and converting from one database to another (or one language to another, etc) is a great way to kill a lot of time and money that could otherwise be spent on actually improving your product.  A better choice, as @Richard Harrison said above, would be to spend some time fixing whatever performance issues are driving you to consider moving to Oracle.  As Fred Brooks said many years ago, there is No Silver Bullet.
Share and enjoy.

A:

It wouldn't be an upgrade to go to Oracle.  Just a migration.  If you wish to upgrade Postgresql I strongly suggest you move to EnterpriseDB's Postgres Plus.  They have created a Oracle compatible version of Postgres that will run all of the existing code, any new oracle centric code.  Nearly all of the features anyone would buy oracle rdms for are in it.  It is really fast too.
If you are jealous of RAC, don't be.  RAC is very expensive to maintain and is not real efficient per new box added.  You can get the additional performance without the cost out of postgres.  Look into PGPOOL as a solution for horizontal growth without the cost. 

