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Is that right? From reading the third document you linked:

"IRC 877A imposes a mark-to-market regime, which generally means that all property of a covered expatriate is deemed sold for its fair market value on the day before the expatriation date."

Unless this document is leaving something out, the gains would not be taxed at the marginal income rate unless they are short term gains. Presumably most people with large amounts of unrealized gains would be paying long term capital gains taxes, which are currently a 20% rate.

I thought this provision was simply to ensure that you didn't make large amounts of wealth in the US then skip out on paying taxes on your gains. That seems very reasonable to me, although perhaps I am missing something.