Adapting a superfish menu to what I want
If you need the menu to render to the full width of the screen make sure the containers the menu is in doesn't have any set widths, and then set the main ul to a 100% width. Superfish comes with a lot of default styles that can be hard to override. If you use the Drupal Superfish module, configure your Superfish block and set the 'Style' to none, and make sure drop-shadow is ticked off. If you don't use the module you can do this when you call Superfish with drupal_add_js (or in your init.js).