Here's the detail on Macau - I actually think it's quite feasible given your timings.

Details
a.) You'll have to take the ferry from HK city if you do Macau first (instead of taking the ferry from HKIA to Macau, since the first one doesn't go until 10am).
b.) Helicopter service: Starts at 9am, runs every 30 minutes. Cost is 2800 HKD each way, takes about 18 minutes from take-off to touch-down. I usually can exit immigration in one port and clear it in the next in around 30 minutes with the helicopter. You get to go through the diplomatic channel at Macau, but coming back to HK there is no immigration benefit.
c.) Ferry service: runs 24 hours, service at intervals of every 15 or 30 minutes between 6am and the evening. Cost is 150 HKD each way for an economy ticket and about 250 hkd for a premium class ticket. Major benefit to the premium ticket (called "super class") is you offload the ferry first, getting you ahead of the mobs for immigration clearance.
d.) Immigration is rarely over 10 minutes in either place. Both immigration facilities aren't exactly modern, but despite the dingy appearances at the ports you can wait a lot longer with a passport at HKIA than you can at the Macau or HK ferry terminals.

Say you arrive at 5:45am. You can easily be on the 7:30am ferry from the HK/Macau ferry terminal in Sheung Wan (main HK ferry terminal). You'll be through immigration in Macau by 9am. You could tool around Macau until 1pm and take the helicopter back, clearing HK immigration no later than 1:30pm. You then could have a 4 hour tour of HK before taking the Airport Express back and getting on the plane again.

If you want to do the ferry returning, you'll need to shorten your tour of HK to about 3 hours.