citizen, you are entitled to $80K exemption for the first $80K earned outside the US. Then, whatever you pay to the HK equivalent of IRS can be used as tax credit against your tax paid on the amount in excess of the $80K. The key tax saving for any American working outside of the US essentially boils down to the initial exemption, which is still considerable if your alternative is to work in NYC or CA where tax burden is quite high, and tax minimization options are very limited for high earning couples who hit the AMT bracket.
Nobody (except for those with a HK passport of course) can work in HK without a visa. But it's much easier to obtain a work visa in HK than H1B in the US, for Americans or Chinese with a GC.
