Chatting to a government official yesterday and he suggested that HKIA expansion for a third runway was not really about extra capacity as there were other options to increase capacity and the projections were optimistic. He suggested that one of the main drivers was the transfer of control of Hong Kong airspace to the Mainland.
The current air traffic hand-over mechanisms between Hong Kong and China are supposedly cumbersome with near distance route constraints and Shanghai determining slot capacity in the busy Eastern corridor.
The undiscussed official line seems to be that it would all be much easier if Hong Kong airspace was no longer separate and the City became a region under the control of Guangdong.