The only issue I can imagine in this scenario is that you are initialising Superfish before the HTML for the menu has been loaded and injected into the DOM. Make sure you are putting the Superfish initialisation code in the callback of the function you are using to dynamically load the XML, and after the code that injects the menu into the DOM.
Superfish is server-language agnostic. It doesn't care what you use on the server, ie. PHP etc. Joel Birch.
