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, the latest version of which you can read at http://www.postgresql.org/docs/current/static/release.html.
This section will help you guide through features and settings that aren't available in older versions (or are removed from newer versions) of PostgreSQL in a more compact form, targeted specifically at performance-related ones. You will need to think first why digression is more important.
