When the Lead is converted to Contact in Salesforce, can it sync back to Drupal?

There's currently nothing in the Suite to handle this. You will need to write a custom module that implements the pre_import and post_export hook. In your post_export hook you'll want something like this (note I've defined constants for my fieldmap names):

function my_module_salesforce_api_post_export($sf_object, $name, $drupal_id, $response) {
  if ($response) {
    if (is_array($response->errors)) {
      $response->errors = $response->errors[0];
    }
    if ($response->errors->statusCode == 'CANNOT_UPDATE_CONVERTED_LEAD') {
      if ($name == MY_SITE_LEAD_FIELDMAP && $drupal_id) {
        // We need to update the object map to reference this as a Contact
        $new_object = salesforce_api_retrieve(array($sf_object->Id), $name);
        if (is_array($new_object)) {
          $new_object = $new_object[0];
        }
        if ($converted_sfid = $new_object->ConvertedContactId) {
          sf_user_export($drupal_id, MY_SITE_CONTACT_FIELDMAP, $converted_sfid);
        }
      }
    }
  }
}

In pre_import hook you'll want something like this:

  // Handle Leads that are converted to contacts
  if ($sf_data->ConvertedContactId && $name == MY_SITE_LEAD_FIELDMAP) {
    // The person has been converted to a Contact but we are using the Lead
    // fieldmap so we need to use the Contact fieldmap for import
    $name = MY_SITE_CONTACT_FIELDMAP;
    sf_user_import($sf_data->ConvertedContactId, $name, $sf_data->Drupal_ID__c);
    return FALSE;
  }