To quote Google "One of the main advantages of the asynchronous snippet is that you can position it at the top of the HTML document. This increases the likelihood that the tracking beacon will be sent before the user leaves the page. It is customary to place JavaScript code in the <head> section, and we recommend placing the snippet at the bottom of the <head> section for best performance."
Any chance this option could be added to Gantry. There are 3rd party Joomla plug-ins that do this but it would save us a little overhead.