It is wrong to think that you will pay 16% tax on a monthly income of 40k. You need a much higher income than that in order to reach the "flat rating" of 16%.
Check
http://www.ird.gov.hk/eng/ese/st_com..._07/stcfrm.htm and you will see that on an anual income of HKD 480,000, with a wife and two kids, you end up paying 27,500 in tax - much less than the seven odd thousand per month which was suggested.