You run into the same problem as means-testing social security and similar ideas -- turns out that income is a very pyramid-like distribution and there's just not a ton of it at the top, relative to the base and the middle. It wouldn't surprise me if 75% of this proposed tax would be collected from the component of salaries below the FICA limit ($118,500 for 2016). Another question is do you consider RSUs to be part of payroll on which tax is due? What about options? These can be 50% or more of compensation in many of these large firms.