Your presupposition is that tax rates are low. Income tax might be "low" but there are many more taxes out there. Much of the money that is hidden, is capital, and not just income. There are many legal ways to move money offshore and not pay taxes on it. I'm not taking a stance on right or wrong there, its just fact. When it becomes cheaper to pay people to hid money, vs pay taxes, you will get lots of people doing it.
Hiding money is not cheap either. We would be better off trying to find a way to fix the issue of how to get taxable money back. Obviously there will be a point where breaking the law is cheaper. But, right now, if you raise the taxes on the top, many will simply hide more. Many may pay the tax, but many wont.
A big issue is that the tax system in the US is so complex, that many just simply dont want to deal with it, and hide their money. I know its a dead horse, but the tax system is set up so that only the wealthy can work around it.
I dont know what the solution is, but simply raising taxes does not seem like it would work out the way you think it will.