VO2 max work is higher intensity than threshold work and whilst you have not explicitly stated you are doing VO2 max work, you have implied it though mentioning the 5 minutes hard, 3 mins rest session, which is classically done at well above threshold and is a very intense session, therefore assuming you are doing what is a classic session (you haven't stated otherwise, and you really ought to have done so if this is the case) the 5 minute efforts will be ABOVE threshold :S i.e. what I am recommending is actually LESS intense than what you are currently doing.