A guide to the major changes in each PostgreSQL version starting with 7.4 is available at the Feature Matrix page: http://www.postgresql.org/about/featurematrix.
Since many performance related changes are more internal, you won't necessarily see them there. The details are only in the release notes, which you can read the latest version of at http://www.postgresql.org/docs/current/static/release.html.
This section will help you guide which features and settings aren't available in older (or are removed in newer) versions of PostgreSQL in a more compact form, targeted specifically at performance related ones. First a digression into why this is more important than you might think is in order.