What I did to see more debug info was edit the following lines:
'message' => t('Credit card payment declined: !text', array('!text' => $x_response_text)),
'message' => t('Credit card payment processed successfully. Approval code: !code', array('!code' => $approved_text)),
to
'message' => t('x_response_code:'. $x_response_code .'<br />x_approval_code:'.$x_approval_code.'<br />xml: '.$xml.'<br />Credit card payment declined: !text', array('!text' => $x_response_text)),
'message' => t('x_response_code:'. $x_response_code .'<br />x_approval_code:'.$x_approval_code.'<br />xml: '.$xml.'<br />Credit card payment processed successfully. Approval code: !code', array('!code' => $approved_text)),