Correct, the capacity of Boeing 787-8 is only about 250 passengers, but it can support CX to expend their worldwide coverage but smaller markets (such as Houston, Manchester, Munich, Montreal...etc) that Boeing 747-400, or Airbus A340-600 will have many empty seats. (If CX plans to expand worldwide). Also, more point to point destinations, e.g. Toronto currently require a refuel at Anchorage.
In the furture, the Boeing 787-10 can carry almost 300 passenger in shorter range. That should be a good replacement for CX of current A330-300 and Boeing 777-200 routes.
Besides, other airlines in HK may choose Boeing 787 since it provides plenty flexibility on various markets.