I don't think it will be a problem as long as things are running on schedule (no strikes, maintenance or weather issues) - 1 hr to get to the city; 3 hrs to eat; 2 hrs to get back. Just don't pick a restaurant too far from the Central or Kowloon stations. And that shouldn't be a problem - The shortest I've done is 9 hrs - 1st trip to HK - and took the CX tour of the city. As nice as the CX lounges are, 5 hrs is too much. Enjoy.