Just to mix things up a bit...

I would add more time in BKK. There is tons to do. If you had a solid week there, you still wouldn't run out of things to do. It's not just a big city, it's the cultural capital of the country.

I would spend a bit less time in Phuket. Although I haven't been to Ko Phi Phi, I presume that it's more of a beachy place and more isolated feeling than the major areas of Phuket. Most of the developed areas of Phuket I found unappealing because they were so tourist oriented that they weren't very Thai-- more beer gardens, Korean barbecues, borscht restaurants, and cantinas than actual Thai places.

As much as I love Thailand and things Thai, I didn't enjoy Chiang Mai all that much. I would give it a pass. If you had to choose between three days in Chiang Mai and three days in Siem Reap, that's a no-brainer: Siem Reap to see Angkor and environs.

You could always visit Macau in a day trip from Hong Kong. There's not much to see there culturally, and what's there is easily covered in a few hours. The massive new hotel-casino-resorts, though, would take some time just to absorb, and that's without any time for gaming. Still, if you got an early start from HK and took a late ferry back, you could do it in a day.