The only places I have seen traffic build up is around where Des Voeux West Road meets Connaught Road and the flyover that goes above Connaught Road. Both tend to clear quite quickly. Once the cab is through the cross harbour tunnel it should be a clear run.
A fall back plan would be to get a cab to Hong Kong station and catch the Airport Express train. You can
check in your bags at HK station up to a day in advance and go straight through to security. Below is a link to which airlines allow you to checkin downtown
http://www.mtr.com.hk/en/customer/se...ckin_info.html