Originally Posted by number_6

You do not go through HKG immigration but you do go through Macau immigration and customs after leaving the ferry and sometimes this can take over an hour (longer than the ferry ride). On return to HKIA you go through an emigration checkpoint which also takes a few minutes (but pretty fast); more problematic is that the ferry from Macau to HKIA has strict checkin times (I took it a couple of years ago and from memory it forced me to be at airport 3 hours earlier than I wanted to be, more time in the F lounge but brutal for your kind of trip).
Where you land at HKIA is also a factor, some gates take 20+ minutes longer to get to the ferry check-in than others (and non-CX BKK flights seem to use some of the worst gates). You'll need to work out all these factors and get them optimal in order to make the schedule work. Finally what is the consequence of missing your flight leaving HKG?