I think it depends on which countries you choose and how many you plan to visit during your stay. Vietnam, Cambodia, Malaysia, and Singapore would be cheaper roundtrips from BKK, Philippines would be cheaper from HKG not just because of geography but because of competition and local demand. I would narrow down the places you want to visit and go to web sites of some BKK based travel agents to sample the fares. Click the Thailand tab from the link below to view BKK based fares from that web site. A few BKK agent links are also listed in the Star Alliance forum in a recent post. Add up your planned roundtrips (see if you can nest a few countries together) and compare it to the Cathay option.
www.tiss.com