The only thing I can come up that's happened since last night was that we ran the nightly vacuum analyze on that database, but I did not change the statistics target.
The answer to your question is no, parameters changes are worse would take effect after a reboot - though most are used on the very next query that runs.
The vacuum would indeed likely account for the gains - there being significantly fewer dead/invisible rows to have to scan over and discard while retrieving the live rows that fulfill your query.
David J.
