Well, since your 20 % ceiling only hold water if you discount all data that doesn't fit it*, assume that the top marginal rate of the personnal income tax is a great indicator of the overall tax policy of the government**, and that Americans are genetically different from Northern Europeans***, I'm going to keep considering it bogus.
As for the 100%, it wasn't mean literally at first (Yes, I know I sound like a GOP lawmaker), but as an exemple because it looked like you were confusing % of GDP and gross revenue. I'm still not sure if you were confused or merely confusing.
Now, I know you meant one cannot capture 100% of output, and I'm not contesting that. But as a matter of fact, GDP is defined as "the market value of all officially recognized final goods and services produced within a country in a given period of time." (That's from wiki) Black markets and tax evasion are, by definition, not part of GDP stricto-sensu.
*Like the fact that government revenue are on a steady raise since 1945 if you include local and state government. I'm pretty sure cops don't take 5% of GDP in speeding tickets.
**Hint: Right now the top marginal tax rate went up by four percentage point, but government revenue aren't going to change by much because the other rates didn't change. And I don't think corporate tax rate went up by four percentage point either.
*** If it's a cultural thing, you can change it, although it does take time. However I wouldn't expect it to stay constrant for 80 years.