Do application leftovers have any impact on performance?
Application preferences and caches will not have any impact on system performance, but if there are leftover startup daemons running, etc., there could absolutely be a performance penalty. There is no way to know for certain how much of an impact there could be. The best advice I can offer is to always check the application developer's uninstall guidelines. Some applications utilize uninstaller applications to ensure all the application support files are removed.