My point is as the tax rate currently stands, it will be really, really, hard for the people at large to not benefit somehow from GDP doubling. The fact that the population would benefit doesn't mean that it is fair, but you can have an unfair situation that helps everyone. Whether it is "a good thing" probably depends on what the alternatives are and what precisely this world look like. For example, a world where a robot owning super rich gets all of the income, pays around half of it in taxes, and the taxes support the rest of the population to sit around comfortably and do nothing probably isn't too terrible. (at least from a fairness/hedonistic perspective)
That 39.6% you mentioned is just federal income tax. There is a whole lot of other taxes, not least state/local taxes. The 1% by and large tend to live in CA/NY/MA/IL, and state taxes will bring that up by a lot. If this hyper rich acts anything like the hyper rich of times past, they will also pay a rather large sum in property taxes.
The issue with marginal vs top tax bracket is purely academic in the situation that you posed - in a world where the 1% is making 198% of current US gdp, their income would be around 10-20 million dollar each. I don't think it matters too much what the tax is on the first 400K is in that world.
I am aware that the Buffett rule didn't actually pass, but I also noted that the discussion in the Buffett rule that it would raise a relatively small amount of money. Going by this graph:
It would appear that the superrich isn't actually that good at hiding their money, being taxed at something around 30% on the federal income tax.