Hot to pass an argument to #ajax['path']
From the docs: 


  It is possible to replace ajax_form_callback() with your own functions. If you do so, ajax_form_callback() would be the model for the replacement. In that case, you would change #ajax['path'] from the default 'system/ajax' and set up a menu entry in hook_menu() to point to your replacement path.


So you need to


Set up a menu callback (looks like you've already done that)
Copy ajax_form_callback() to that path's page callback function
Make the changes you need to. The function is very short so it should be obvious which bits you need.
In your form elements use this:

$form['foo'] = array(
  ...
  '#ajax' =&gt; array(
    ...
    'path' =&gt; 'path/to/new/callback',
  ),
);



You shouldn't write any JS code to invoke this manually, Drupal takes care of that for you.

Note that this method does load the full form; that's unavoidable using Drupal's built-in AJAX functionality.